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.
Cleanup logging.
Merge pull request #116 from EasonLiao/plain_epoch

ack_epoch and proposed_epoch are not human readable
Merge pull request #122 from EasonLiao/clean_log

log message cleanup
Merge pull request #129 from EasonLiao/more_tests

More tests for reconfiguraion.
Merge pull request #130 from EasonLiao/remove_enum

Remove enum in Transaction.