Using #4219 as an example.
Instead of starting new goroutines or directly processing messages
in a blocking way, messages are added to a queue that a worker pulls from.
The Pusher engine still currently implements network.Engine rather than
network.MessageProcessor.

Because the event processing now happens in a worker, any errors
raised within it are no longer visible to the caller of Process().
Because the test checked for error status, moved the tests to the
same package and call the internal processing function directly.

When using Unit, calling Ready would also start the engine.
With ComponentManager, we additionally need to invoke Start.

Rename `inboundMessageWorker` and `processInboundMessages` to `outbound`
and also propagate errors to the top level of the worker
where they can be thrown.

- Partially implement suggestion #6747 (comment)
  - Make `SubmitCollectionGuarantee` non-exported and rename to `publishCollectionGuarantee`
  - Add new `SubmitCollectionGuarantee` exported function that just adds to the queue
- Remove `messageHandler` field, instead directly add to queue
  from review: #6747 (comment)
- `OriginID`s no longer included in messages in the queue, and therefore not
   checked by the worker - if necessary they should be checked when Submitting

This reverts commit 0aadc13.
Instead of testing the internals, test the exported interface.

…gine-use-componentmanager

Rename queue and add length metrics for it, updating creation sites.

Co-authored-by: Alexander Hentschel <[email protected]>

Doc comment changes, metrics naming, and queue length.
For reasoning behind chosen queue length, see #6747 (comment)

Co-authored-by: Jordan Schalm <[email protected]>