ws-server: Handle soketto::Incoming::Closed frames

[lexnv]

Ensure that incoming Closed frames are not ignored.

This PR captures the close reason for the connection when detected by the receiver.

While at it, the WS error log line was polluting internal metrics when clients disconnected suddenly. Have changed it from warning to debug.

Logs

• obtained with a custom client that closes its sender part in the background_task

2022-07-06T15:21:18.739200Z TRACE soketto::connection: e0000f78: recv, incoming CLOSE: CloseReason { code: 1000, descr: Some("") }
2022-07-06T15:21:18.739225Z DEBUG jsonrpsee_ws_server::server: WS transport: remote peer terminated the connection: 1

Closes #813 .

[niklasad1]

great but I think one of the logs is sufficient.

what about removing https://github.com/paritytech/jsonrpsee/blob/master/ws-server/src/server.rs#L407?

[lexnv]

Have removed the close reason log instead. I was able to force the MonitoredError::Selector(SokettoError::Closed) code-path with multiple client requests and dropped connection, without triggering the soketto::Incoming::Closed path. The close reason is also printed by soketto for trace-level logs.