Separate directory for each SyncProposalProcessorTest
Merge pull request #135 from EasonLiao/fix_sync_test

Separate directory for each SyncProposalProcessorTest
Merge leading/following callback with ClusterChange callback.
Merge pull request #134 from EasonLiao/callback_change

combine clusterChange and leading/following callbacks?
Take snapshot in broadcasting phase and use snapshot for recovery.
Merge pull request #136 from EasonLiao/snapshot

Take snapshot in broadcasting phase and use snapshot for recovery.