votr: Add VOTR demonstration workload

This change adds a demo workload that shows how the merge function can be used
to implement asynchronous, two-way and hub-and-spoke replication. A more
complete discussion of the workload is provided in its README file.

A small extension to the existing parse-helpers API is necessary to ensure that
JSONB values are reified as json-ish types when loading conflicting rows from
the database. The `server.Server` type also supplies its listener address so
that VOTR can automatically create changefeeds.

internal/cmd/votr/README.md

@@ -0,0 +1,113 @@
+# VOTR
+
+The VOTR workload demonstrates a hub-and-spoke replication workload that
+uses cdc-sink's merge features.
+
+The core workload demonstrates a ballot-casting system. There are three tables:
+* `candidates` is an infrequently-updated table that uses a version
+  column and last-one-wins behaviors
+* `ballots` is an append-only table with a composite primary key of
+  `(candidate uuid, ballot uuid)`
+* `totals` maintains per-candidate running totals. It is intended to
+  show how a high-contention table can be logically replicated; this is
+  not an ideal pattern for CockroachDB.
+
+VOTR shows a hub-and-spoke replication model, but it can also operate in
+a two-datacenter (2-DC) model. The `votr_0` database is used as the hub,
+and an arbitrary number of "spoke" databases, `votr_N` run the workload.
+Each hub-spoke pair has two unidirectional changefeeds, each of which
+are processed by independent `cdc-sink` instances.  There are a total of
+six independent feeds that are operating in this demonstration.
+
+![votr-diagram.png](votr-diagram.png)
+
+The worker attached to each spoke selects a random number of candidates
+and casts one or more ballots to each of them. The VOTR worker knows
+nothing about the replication model. The workload is roughly:
+* `BEGIN`
+* For random Candidate and Number of votes:
+  * `UPDATE totals SET total = total + N WHERE candidate = C`
+  * `INSERT INTO ballots (candidate) VALUES (C, generate_series(1, N))`
+* `COMMIT`
+
+Each spoke also has a validation loop that checks whether or not the
+total number of ballots equals the running total of all totals. That is
+`SELECT count(*) FROM ballots` should equal `SELECT sum(total) FROM
+totals` at any given point in time.
+
+The only SQL client to interact with the `votr_0` hub is cdc-sink.
+
+The tables used by the `VOTR` workload are augmented with a vector-clock
+scheme. That is, each table has a `whence JSONB` column that shows which
+worker(s) a row incorporates state from. A `src INT` column also allows
+the `vectorMerge()` function defined in the [votr.ts user
+script](./script/votr.ts) to ensure idempotency by ensuring that the
+clock for a particular source only ever rolls forwards.  Note that the
+functionality does *not* depend on clock synchronization between the
+workers, only that each worker has a reasonably well-behaved cluster
+timestamp.
+
+# Running the demo
+
+Start one or more local CockroachDB instances. The VOTR workload can use
+multiple SQL databases on a single CockroachDB node, or the databases
+can be split across multiple clusters.
+> `cockroach start-single-node --store type=mem,size=2G --advertise-addr 127.0.0.1:26257  --insecure `
+
+Initialize the VOTR schema on each instance. The number of regions is
+specified by repeating the `--connect` argument. It is valid to have
+duplicate connection strings to use the same CockroachDB node or cluster
+to host multiple VOTR databases. This command-line shows the local
+CockroachDB node being used to host the hub and three spokes.
+> `cdc-sink votr init
+> --connect 'postgresql://root@localhost:26257/?sslmode=disable'
+> --connect 'postgresql://root@localhost:26257/?sslmode=disable'
+> --connect 'postgresql://root@localhost:26257/?sslmode=disable'
+> --connect 'postgresql://root@localhost:26257/?sslmode=disable'`
+
+Run the workload. The `--drainDelay` flag sets a timeout after receiving
+a `^C` for the workers to stop, but to allow the cdc-sink instances to
+continue to process messages.
+> `cdc-sink votr run
+> --candidates 128 --workers 1 --drainDelay 1m
+> --connect 'postgresql://root@localhost:26257/?sslmode=disable'
+> --connect 'postgresql://root@localhost:26257/?sslmode=disable'
+> --connect 'postgresql://root@localhost:26257/?sslmode=disable'
+> --connect 'postgresql://root@localhost:26257/?sslmode=disable'`
+
+# Useful demo query
+
+Show the count of ballots and sum total across multiple VOTR database on
+the same node:
+```sql
+WITH t (n, t) AS (                                         
+                     SELECT 1 AS n, sum(total)             
+                       FROM votr_1.totals                  
+                     UNION ALL SELECT 2 AS n, sum(total)   
+                                 FROM votr_2.totals        
+                     UNION ALL SELECT 3 AS n, sum(total)   
+                                 FROM votr_3.totals        
+                     UNION ALL SELECT 0 AS n, sum(total)   
+                                 FROM votr_0.totals        
+                ),                                         
+       c (n, c) AS (                                       
+                     SELECT 1, count(*) FROM votr_1.ballots
+                     UNION ALL SELECT 2, count(*)          
+                                 FROM votr_2.ballots       
+                     UNION ALL SELECT 3, count(*)          
+                                 FROM votr_3.ballots       
+                     UNION ALL SELECT 0, count(*)          
+                                 FROM votr_0.ballots       
+                )                                          
+SELECT *                                                   
+  FROM t JOIN c USING (n) order by n;
+```
+
+# Known limitations
+
+* VOTR is a one-shot demo since the changefeeds are deleted on exit. The
+  cdc-sink HTTP ports are ephemeral, so it's not straightforward to
+  continue from a previous run. With more development effort, VOTR could
+  query the jobs table to determine which port it should attempt to bind
+  a listener to, in order to use the existing state.
+* VOTR should demonstrate adding a new region.