[realtime] Multiple websocket are open

[gregorylegarec]

Cozy-Realtime open several sockets to the stack instead of only one.

This is due to race condition located in getCozySocket. If several calls to realtime.subscribe or subscribeAll are made in a row, each will generate a new call to getCozySocket. But as getCozySocket is asynchronous, the socket singleton will not be return and a new condition will be made.

Instead of the cozySocket, we should return here the promise of the cozySocket.

This fix need tests.

[gregorylegarec]

Thanks @nono for spotting this race condition 👍