Use async for sending over websockets

[koch-t]

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

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