Client improvements

- only used connMutex around operations that modify the connection
- readLoop and writeLoop exit if conn is nil
- moved writeBuf var inside of writeLoop
- removed isConnected (conn != nil should be the same test)
- no coordination in readLoop and writeLoop
- pipe go-steam/client log output through emit so we can have context in logging
- label Fatalf transient errors as such, should be much simpler now
- lowercase all log messages for easy gripping