Adds the `server.add_peer` and `server.peers.subscribe` RPC commands to
support discovery of other servers, with a mechanism for running
scheduled health checks and verifying the servers availability.
The electrum protocol does not support paging, which makes this a potential DoS
vector if no limit is set.
The limit is configurable via --electrum-txs-limit and defaults to 100.
A similar change was made in upstream electrs:
e1825fdc77
