Use ParticipantState object to share states among different instances of Participants.
Merge pull request #114 from EasonLiao/master

Use ParticipantState object to share states among different instances of...
Change quorumSet from HashMap to ConcurrentHashMap.
Merge pull request #120 from EasonLiao/fix_quorumset

quorumSet of leader should be ConCurrentHashMap instead of just HashMap
Refactor the QuorumZab interface.
Merge pull request #115 from EasonLiao/refactory

Clarified workflow of client interface.
Add state change callback.
Merge pull request #118 from EasonLiao/clarification

Server state MDC doesn't get set when starting a single server?
Buffer requests before broadcasting phase.
Merge pull request #110 from EasonLiao/buffer_requests

Participant should buffer send request before broadcasting phase.
Add type field to Transaction.
Add PersistentState test.
Make reconfig logic more clear.
Change from LEAVE to REMOVE on reconfig.
Some comments and cleanup
Merge pull request #127 from EasonLiao/cop2log

Append COP to log and some refactory on reconfig logic.
Merge pull request #111 from EasonLiao/persis_test

Add unit test for PersistentState class.
More tests for reconfiguraion.
Made ack_epoch proposed_epoch file readable.
Remove enum in Transaction.