Gateway spawns unnecessary tasks

[Bluenix2]

Description

Currently the gateway spawns two tasks to be able to use asyncio.wait() to receive an event:

• A task waiting for the close event
• The other task waiting for the WebSocket

This is unnecessary, as only the latter task needs to be re-created really.

Steps to fix

1. Add an optional (potentially None) attribute to __slots__ and initialize it as None
2. In the receive() (or if it was the run()) method check for this task and use it in asyncio.wait(), if it doesn't exist create it.
3. Edit the code to stop cancelling the task when the WebSocket receive suceeds