Use async for sending over websockets

The implementation prior can heavily suffer from the slow subscriber issue where one client can possible slow down the other clients and even the filewriter.

NOTE:
I'm not sure if this change, sending over websockets async doesn't allow for new issues such as dead sockets that will never be closed or huge backlogs on one client.
Maybe a zeromq fan-out model where each client works on it's own queue is more reliable, this would also reduce locking issues when adding new subscribers, as each client will work on a different thread.
Using a ZeroMQ poller can also implement a heartbeat which currently is not implemented yet implied by the header