Remove Null Failure Detector - no longer used as all relevant test cases are better done with the real thing.
Make access to user-code identifiers associated with Paxos node information easier/cleaner.
Documentation for HeartbeaterImpl
Fix tests to implement new FailureDetector contract.
Make user-code node identifiers available in state transitions to better support common usage patterns.
Fix up a comment in FailureDetectorImpl
Clean up recovery process in Backend
Remove dead imports.
Clean up access modifiers
Remove dead logger in Common.