Add message bus for asynchronous updates

[kengu]

We need to add better support for message-driven updates of front-end views and data. Long pulling which we use today is expensive with respect to number of connections and workload and should only be used as a fallback for modern techniques like server-sent events. The front-end should be able to subscribe to messages which when received automatically updates front-end views and data.

Solution

• Add message bus to back-end using ZeroMQ
• Add message dispatcher to front-end

References

• Web-transports and fallbacks