Websockets - filtering options

[guilhermelawless]

This adds per-subscription filtering options via an extra field in a subscribe action:

{
  "action": "subscribe",
  "topic": "confirmation",
  "options": {
    "all_local_accounts": "true",
    "accounts": [ "xrb_123...", "..." ]
  }
}

Filtering happens after the message is built and is sent for broadcasting, with a bunch of O(1) checks for existance of accounts. We would have to change the dynamics of the websocket server quite a bit to handle it differently. Open to suggestions as usually or just pickup from this PR and improve!

Can add something to core_test if this seems ok.

Related: #1901

[cryptocode]

#1906 is merged so this one will need a rebase @guilhermelawless

[cryptocode]

Could we get a test for confirmations being filtered / not filtered for both query options?

I'm adding the documentation tag so we remember to update https://github.com/nanocurrency/nano-node/wiki/WebSockets once merged.

[guilhermelawless]

Some tests were failing due to the destination being in block.destination and not block.link_as_account for some blocks.

Other tests were incomplete or some things untested due to not using a true async read on the socket client test.

Everything should be ok now.

[cryptocode]

@guilhermelawless Thanks! The Websocket Wiki page needs an update, feel free to attach markup here.

[guilhermelawless]

@cryptocode md is not supported so i zipped it, here you go!

WebSockets.zip

[cryptocode]

Docs merged into the Websockets wiki page