v0.22.0: Block rewards, improved Flash API & private channels

Major improvements since v0.21.0:

Flash nodes now use separate registry endpoints

Flash nodes should now register themselves via the POST /flash_node endpoint,
and can be accessed via the GET /flash_node endpoint of the registry,
instead of using the /validators endpoint as before,
which is now only used for enrolled validators.

Registering a validator in the registry now uses POST

Before, the registry was expecting a PUT /validators request to register an address,
however that was at odds with the method definition.
Validators now need to use a POST request.

Removed recv_pk parameter from FlashNode.openChannel API

This parameter was redundant with the content of the chan_conf parameter
and shouldn't be provided anymore.

Flash: openChannel now properly validates funding TXs

The openChannel call of the Flash API was previously missing some validation
on the funding transaction: invalid funding transactions will now be properly rejected.

Flash: ThinFlashNode & AgoraFlashNode have been removed

The flash nodes have been unified under a single type, FlashNode,
providing more flexibility by better defining its relationship to Agora.
As part of this, a new parameter, funder_address, is now required for openChannel,
and peer_address is required for FlashControlAPI.openNewChannel.

FlashControlAPI is now a BlockExternalizedHandler

FlashControlAPI now implements the BlockExternalizedHandler API as well,
which allows one to put a Flash node in the event_handler.BlockExternalized section.

The fee check now properly checks the combination of data and transaction fee

Previously, the fee checker mistakenly checked both fee individually,
instead of the combination of both.

The accumulated_fee table has been replaced by block_fees

The old table has been dropped, and the new table is used to store block fees
to be paid in the next block reward between payout period.

FlashControlAPI.openNewChannel: Removed redundant reg_pk

This parameter has been removed, as it is already provided by funding_utxo.address,
and should not be provided anymore.

Block rewards are now included in Coinbase transactions

From this release, block rewards are now properly distributed every payout period.
A payout period is currently set to 144 blocks (1 day).

When the payout block is reached (height % 144 == 0 and not genesis),
a coinbase transaction is added to the block, that contains the data fees,
transaction fees, and confirmation rewards for the period before last.
This means that payout for the range [0; 143] will happen at block 288,
payout for range [144; 287] happens at block 432, etc...

Block rewards are distributed according to the amount set in the whitepaper,
and proportional to the number of blocks a validator has signed during the period.

Registry: TCP queries are now supported

The network registry now also listens to the TCP port (53 by default) in addition of the UDP one,
meaning that it might need to run privileged under some systems.
This will allow zone transfer and large payload to be exchanged.
Note that systemd users might need to bind to the public interface only,
as systemd-resolvd binds to the loopback interface's port 53.

Enrollments no longer include cycle_length

Cycle length was originally included in Enrollment to allow nodes to configure
the duration of their enrollments. However, it had turned into a consensus-controlled
parameters since the inception of Agora, and hence wasn't used anymore.

Registry: Reworked zone configuration

Registry configuration has been simplified:

• The registry.dns section has been folded directly into registry;
• The registry.[dns.]authoritative section has been replaced by registry.validators
  and registry.flash, and an authoritative boolean value has been added to them;
• registry.validators controls the zone for validators (nodes that are enrolled);
• registry.flash controls the zone for Flash nodes (nodes with a channel open);

Node configuration now includes a realm field

In order to support the new function of the registry, the configuration has a new option: node.realm.
This field controls the root domain under which the validators and flash zones can be found.
Available realms at the moment are: testnet.bosagora.io and coinnet.bosagora.io.

Flash: Channels can now be private

The FlashControlAPI.openNewChannel endpoint now accepts a is_private boolean as a parameter.
If set to true, the channel will not be gossipped, and requests to use it for routing will be denied.
This allows users to keep their channels unknown to the rest of the world, further privacy.