Create new websocket function websocket_check_pingpong

[danidee10]

This adds a new method called websocket_check_pingpong #1717

Right now, It's merely a wrapper around uwsgi_websocket_recv_do (like websocket_recv and websocket_recv_nb) and the non-blocking behaviour is hardcoded.

I tried to call the C function uwsgi_websockets_check_pingpong but I got errors and since the internals of uWSGI isn't documented, I didn't really know what to do. I believe some things must be done on the wsgi_request (e.g uwsgi_websocket_parse_header(wsgi_req);) before trying to send a ping or receive a pong.

The only solution was to copy most of the code from uwsgi_websocket_recv_do but that doesn't seem right.

I'm submitting this PR (Not to be merged in its current state) because I don't really know how to proceed and how to handle non-blocking and blocking mode that's an argument to uwsgi_websocket_recv_do (It's only used in one place in the function body so I feel it can easily be removed) .

Obviously there are no tests yet 😄

[danidee10]

This is loosely related to #1533 because with the new function I still experience the same No PONG received in 3 seconds and currently the only workaround is to kill the uWSGI process which forces it to restart (provided uWSGI is started with it's --master process). and on the client side try and re-establish a connection.

If there's a separate function for handling pings and pongs it'll be easier to solve the problem since we won't be debugging code that has to deal with receiving of packets.

[maxim-s-barabash]

the proposed solution #1261