Split control flow from data processing in sumeragi

[mversic]

At the moment we have only one control flow message in sumeragi, namely ViewChangeSuggested, and it is processed alongside other data messages. This is not optimal for many reasons. One of the problems is that they share the same queue which introduces a lag in the processing of control flow messages due to which under high contention(lot of gossiping, etc.) nodes can stay out of sync for prolong periods of time

Additionally, control flow messages should also have priority in processing because some of the data messages waiting in the queue (that have arrived before the control message) could already be outdated.

This separation can be achieved with a separate queue for control flow messages or by changing the existing message queue into PriorityQueue where control flow messages have a higher priority than data messages