update dependency for 0.6.0 deb package
improve log messages
improve exception message
Merge pull request #822 from jubatus/improve-logs

Improve logs and exception messages in server
Merge pull request #817 from jubatus/deb-0.6.0

update dependency for 0.6.0 deb package
unregister membership from ZK before stop server
append `close_lock_service()` for `prepare_for_stop()`