Implemented websocket /feed on HTTP (REST) API

[vtnerd]

This is marked as a draft until the client implementation is done. I want to make sure the protocol works before even merging this code.

This design adds a /feed endpoint that is not REST, instead the endpoint is a web socket push/stream protocol. The user logs in via address + viewkey, then automatically gets the entire state of transactions from the server. The schema differs from the existing REST api in that the minimum amount of data to support the wallet_api.h is sent (the existing REST API had lots of data redundancy).

After that initial sync, the /feed interface sends "raw" scanner information to the frontend, via ZMQ internally, including mempool matches and all newly scanned blocks. Reorgs and monerod disconnects are also reported to the clients. If the /feed threads are overloaded and miss internal ZMQ messages, this is detected and the stream errors out.

The design supports both JSON and msgpack, the latter being ~10% faster in prior tests.

There are some basic tests, but more would be useful. At this stage I'd rather get a real client so that I can test viability of the design. There will also be a markdown explanation of the final protocol.

[vtnerd]

I'm going to "rebrand" this as /feed to avoid confusion with web push type stuff. The REST endpoint for establishing a web push will likely be /push instead.

*if ever implemented, as its more complex than /feed

[vtnerd]

Renamed just about everything from "/push" to "/feed". The former is basically reserved for w3c "push" messages to clients (which may or may not be implemented).

This also simplified pushing/writing such that a separate write "loop" is easier. This was made possible by leveraging builtin boost::beast::websocket::stream timeout functionality.

[vtnerd]

Forgot to mention that I added suppression of some block scan messages, primarily when the backend is actively scanning old blocks of an account.

[vtnerd]

Force pushed some tweaks to the protocol after implementing the lwsf (frontend) portion. These changes reduce the fields to the bare minimum, and mostly appear working.

A separate issue for "null" tx-hashes was discovered and will be patched separately.

[vtnerd]

Mostly renames with a few bug fixes. This matches what the client is now doing.

[vtnerd]

Rebased and added docs with schemas for /feed

[vtnerd]

A critical fix for stability, caught with ASAN.

[vtnerd]

Tweaked the /feed schema. The id field of spends are no longer required. This is needed for FCMP++ legacy and carrot view-incoming accounts as the backend can only infer a spend through a CHANGE tx (the output being spent cannot be known).