Event loop for pipeline processing

[lebdron]

Description of the Change

Create a synchronize subject in consensus to queue the events from the network without locking the operation of consensus

• Patch rxcpp to remove data race and allow using current_thread scheduler with synchronize
• Add subscriptions for subjects in irohad to manage their lifetime
• Add round parameter to Yac to close rounds that are less than or equal to current round
• Add publish().ref_count() to YacGateImpl observable to eliminate redundant calls to flat_map if multiple subscribers are present
• Fix data race in getSpdlogLogLevel
• Fix nondeterminism in consensus_sunny_day test 🎉
• Allow usage of blocking_observable in TestSubscriber

Benefits

Consensus can process messages from the network while the pipeline is running

Possible Drawbacks

A separate thread is allocated to the pipeline

[luckychess]

CI confuses me a bit - looks like all tests are passed but it shows failure status.

[MBoldyrev]

also it would be nice to give the patch patch/synchronize-lock-recursion-thread-safety.patch a more meaningful and descriptive name. I would suggest either renaming it with some rxcpp prefix or moving to a directory like patches/external/rxcpp (it other patches to rxcpp are probable).