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.