feature request: ping / pong

[eislambey]

for detecting broken connections we can use ping / pong mechanism.

If the WebSocket connection supports ping & pong (i.e. advertises itself as draft 01 or above), Channels will send ping messages to the client in order to verify that it is active. In protocol versions 5 and above, when using an old version of the WebSocket protocol, Channels will send pusher:ping event (see events to the client). The client should respond with a pusher:pong event.

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