client: Implement notify_on_disconnect

[lexnv]

This PR introduces the client's notify_on_disconnect functionality.

When called, the method completes when the client is disconnected
or the client's background task encountered an error.
The method is cancel-safe, as the client disconnects or background task
errors are unrecoverable.

Testing Done

• Server: Started a process with WS server that terminates in 60 seconds
• Client: Submitted a request (to verify connectivity) and called client.notify_on_disconnect().await;

Logs

Server

2022-08-02T10:00:37.479952Z DEBUG jsonrpsee_ws_server::server: recv pong
2022-08-02T10:00:37.601751Z  INFO ws_server: Server Shutdown

Client

2022-08-02T10:00:37.604050Z ERROR jsonrpsee_core::client::async_client: Error: Connection(Closed) terminating client
2022-08-02T10:00:37.604176Z  WARN jsonrpsee_core::client::async_client: Transport(WebSocket connection error: connection closed

Caused by:
    connection closed)
2022-08-02T10:00:37.604617Z TRACE soketto::connection: ebce45eb: closing connection
2022-08-02T10:00:37.604655Z TRACE soketto::connection: ebce45eb: send: (Close (fin 1) (rsv 000) (mask (1 33031046)) (len 2))
2022-08-02T10:00:37.605058Z TRACE soketto::connection: ebce45eb: Sender flushing connection
2022-08-02T10:00:37.605205Z TRACE mio::poll: deregistering event source from poller
2022-08-02T10:00:37.605375Z  INFO ws_client: Client stop...

Next

• Handle tokio::SendError and futures_utils::SendError conversion
• Handle WS part

Closes #770.

[niklasad1]

this is really great, we should adopt this idiom IMO to document cancel safety.

most things here are cancel-safe but really great to be explicit.

[jsdw]

Agreed! It's so useful to know :)

[jsdw]

One other thing; can we add a couple of tests for this new function?

[niklasad1]

no more tokio::sync::Notify?
doesn't it compile for WASM or what's the reason?

[jsdw]

This perhaps?: #837 (comment)

[lexnv]

Yep, that's the explanation, mainly to reduce dependencies on tokio 🙏