`jest-websocket-mock` only support a single client connection

[piranna]

At https://github.com/romgain/jest-websocket-mock/blob/cf0695a30d78dda0bd9e503753fb37124f38eccd/src/websocket.ts#L49-L50 are defined _isConnected to be resolved when a new connection is established, and _isClossed when the WebSockets server gets clossed. I think there should be a mechanism to be notified each time a new connection happens, so it's possible to have multiple clients at the same time.