線程同步機制: Locks, RLocks, Semaphores, Conditions, Events和Queues
http://yoyzhou.github.io/blog/2013/02/28/python-threads-synchronization-locks/
http://xiaorui.cc/2015/07/10/%E6%89%AF%E6%89%AFpython%E7%9A%84%E5%A4%9A%E7%BA%BF%E7%A8%8B%E7%9A%84%E5%90%8C%E6%AD%A5%E9%94%81-lock-rlock-semaphore-event-condition/