ws-server: Submit ping regardless of WS messages

[lexnv]

This PR ensures that a Ping frame is submitted regardless of WS traffic.

Previously, the ping interval timer was dropped with each iteration of the select loop.
As a result, we could encounter a scenario of Ping starvation, where pings are never
submitted due to WS traffic being received at a fast pace.

This ensures the ping interval timer is propagated and reinitialized when triggered, leading
to a Ping being submitted even if the server is handling messages.

One drawback of this approach is that the server spends some cycles handling the Ping
frames instead of the actual server load. One advantage is that the keep-alive functionality
is entirely handled via control frames, as opposed to either: ping interval being expired, or
WS messages handled.

Testing Done

• manual testing performed with ws server and ws client
• ws server registered to submit pings every 2 seconds
• ws client submits requests every 1 second

> ping - 1
2022-06-02T12:12:52.781744Z DEBUG jsonrpsee_ws_server::server: send ping
2022-06-02T12:12:52.781967Z TRACE soketto::connection: 3918d5d9: send: (Ping (fin 1) (rsv 000) (mask (0 0)) (len 0))
2022-06-02T12:12:52.787939Z TRACE soketto::connection: 3918d5d9: recv: (Pong (fin 1) (rsv 000) (mask (1 7e9b6b21)) (len 0))
2022-06-02T12:12:52.788033Z DEBUG jsonrpsee_ws_server::server: recv pong

> request - 1
   2022-06-02T12:12:52.788106Z TRACE soketto::connection: 3918d5d9: recv: (Text (fin 1) (rsv 000) (mask (1 bfcc4e07)) (len 45))
   2022-06-02T12:12:52.791014Z TRACE jsonrpsee_ws_server::server: send: {"jsonrpc":"2.0","result":"lo","id":3}

> request - 2
   2022-06-02T12:12:53.798444Z TRACE soketto::connection: 3918d5d9: recv: (Text (fin 1) (rsv 000) (mask (1 daff6382)) (len 45))
   2022-06-02T12:12:53.798858Z TRACE jsonrpsee_ws_server::server: send: {"jsonrpc":"2.0","result":"lo","id":4}

> ping - 2
2022-06-02T12:12:54.783803Z DEBUG jsonrpsee_ws_server::server: send ping
2022-06-02T12:12:54.784021Z TRACE soketto::connection: 3918d5d9: send: (Ping (fin 1) (rsv 000) (mask (0 0)) (len 0))
2022-06-02T12:12:54.788594Z TRACE soketto::connection: 3918d5d9: recv: (Pong (fin 1) (rsv 000) (mask (1 6664414f)) (len 0))
2022-06-02T12:12:54.788635Z DEBUG jsonrpsee_ws_server::server: recv pong

[TarikGul]

One drawback of this approach is that the server spends some cycles handling the Ping
frames instead of the actual server load. One advantage is that the keep-alive functionality
is entirely handled via control frames, as opposed to either: ping interval being expired, or
WS messages handled.

👍

[niklasad1]

great, some questions if we could use tokio::interval instead of tokio::sleep
also maybe it makes sense to have this disabled by default and folks can opt-in if they want this overhead.

we will likely catch this from the "daily" benchmarks anyway.