Add random sleep in round robin election.
Merge pull request #80 from EasonLiao/random_sleep

Add radomized sleep to break round-robin loop
Add state change callback to StateMachine
Merge pull request #78 from EasonLiao/pending_txns

Deliver transactions in initial history before broadcasting phase
Merge pull request #58 from EasonLiao/state_cb

Add zab state callback (LOOKING/LEADING/FOLLOWING)
Avoid sending duplicate COMMIT messages.