Features: Websockets, Public query of message without data

[otobongfp]

• Client-side tracks message hashes and checks relay for new message via web-socket.
• Use and address and hash to get the metadata of a transaction without the data.

[jasny]

Relay should be able to effortlessly serve many client apps (wallets). The overhead of holding many connections open is unnecessary since new messages are infrequent (this is not a chat app).

Instead use the If-Modified-Since header so the server can respond with 304 Not Modified.