Improve websocket connection

[macartur]

This commit will improve the websocket to use gevent + gevent-websocket to avoid
the error KeyError: 'Session is disconnected'
Fix #416

[cemsbr]

I tried to use eventlet, but it was not logging messages sent outside the eventlet. Did you check whether all messages are being sent?

Besides, I'm working on a huge logging refactoring removing the websocket buffer and I think we should wait my PR to discuss a possible merge (in the next day or two).

[cemsbr]

My comments are in the conversation.

[beraldoleal]

Since that @cemsbr is working on this I will wait until his PR. Also I noticed two more dependencies. We have to try to avoid adding more dependencies.

[cemsbr]

To have WebSockets instead of long polling, we must use gevent (and gevent-websocket) or eventlet in flask-socketio. Both lead to strange behavior when logging from outside flask. This is because they make flask run asynchronously, and our code is not async. I haven't found a simple solution for that.

I suggest keeping the long-polling approach until we change our architecture. Performance is not an issue right now.

[beraldoleal]

@macartur ?

[diraol]

@cemsbr this PR should be considered even after your PR about this?

[cemsbr]

I'm closing this in favor of #451, as discussed with @macartur.