Make consensus state machine asynchronous (BFT-406)

[moshababo]

Goals

• leader being unable to block colocated replica (and vise versa).
• Having an inbound message queue with the capability to invalidate/remove messages from the same validator.

Main changes

• sync::prunable_mpsc module has been added to provide a channel-like FIFO queue with additional functionality for removal of specific pending values.
• Introduced one background task for each state machine (replica/leader), which consumes corresponding messages from the inbound queue. Top-level processing includes only send() operation, without waiting for the processing response.

Follow-up:

• Consider additional asynchronous pipelines within each role
• Re-evaluate caching and transitioning within the state machines

[pompon0]

approach lg in general, I've added comments about conventions and expected usage of structured concurrency. The important comment is the one about pruning semantics which won't work as is, but should be easy to fix.