fix: close read loop before closing websocket fix #1530

[pcriadoperez]

Problem

When closing the BinanceWebsocket in some cases the websocket will close before the readloop, causing the read loop to return to the user a ClosedConnectionError.

Solution

• When closing the binanceWebsocket, first the read loop is exited and the websocket is stopped.
• Remove unnecesary line of setting the ws state to Exiting as it's done already inside _kill_read_loop

[coveralls]

coverage: 68.945% (-0.02%) from 68.96%
when pulling 27e5772 on pcriadoperez:1530
into d8e2ce1 on sammchardy:master.