Multi address tracking

[mononaut]

(builds on #4209)

This PR adds support for subscribing to multiple addresses and scripthashes over the websocket.

The behavior of the existing track-address subscription is unchanged.

The PR adds two new subscriptions:

• 'track-addresses', which expects an array of addresses/pubkeys as payload.
• 'track-scriptpubkeys`, which expects an array of hexadecimal scriptpubkeys strings as payload.

Each client may have one active subscription of each type (track-address, track-addresses, and track-scriptpubkeys). Subsequent requests replace open subscriptions of the same type.

Sending a track-addresses or track-scriptpubkeys request with an empty array payload ([]) ends the current subscription.

Relevant events are passed to subscribed clients under the multi-address-transactions and multi-scriptpubkey-transactions keys respectively, in the format:

{
  ...
  'track-address-transactions': {
    <address-1>: {
      mempool: MempoolTransactionExtended[],
      confirmed: MempoolTransactionExtended[],
      removed: MempoolTransactionExtended[],
    },
    <address-2>: {
      mempool: MempoolTransactionExtended[],
      confirmed: MempoolTransactionExtended[],
      removed: MempoolTransactionExtended[],
    },
    ...
  }
  ...
}

where for each subscribed address, mempool is a list of relevant new transactions entering the mempool, confirmed is a list of transactions included in a new block, and removed is a list of transactions evicted from the mempool.

Note that transactions may be confirmed without a corresponding prior mempool event.

[softsimon]

I can see how this can be abused with no limits. Perhaps there needs to be a backend config with a default limit of 10 (or something) maximum subscriptions per client, with the possibility to raise the limits on servers dedicated to enterprise customers.

[mononaut]

I'm not sure this is any more vulnerable to abuse than individual subscriptions, and might actually be less so since it reduces the overhead of maintaining different websockets, although I suppose it depends how effective our rate limiting is for websocket initiations.

We also improved performance and scalability quite a bit in #4095.

[mononaut]

rebased on current master

[mononaut]

Added a config.MEMPOOL.MAX_TRACKED_ADDRESSES config setting, with default 1.

Attempts to subscribe to more than that number of addresses or scriptpubkeys will result in an error message returned in the websocket response object.

[CharliesAngel1]

approve

[mononaut]

Fixed bad error response and rebased on master

[softsimon]

tested ACK @ [0cddfcf]