cpp: Let Monitors share Mutex instances

cpp: Let Monitors share Mutex instances

- Let Monitor objects share a Mutex() instance so that more than one
  condition can be implemented on top of a single mutex protecting an
  important data structure.
- Make Mutex and Monitor noncopyable
- Add an accessor to Mutex() so the underlying pthread_mutex_t* can be
  retrieved for passing to pthread_condwait
- Change Monitor to use the actual Mutex class instead of creating a
  naked pthread_mutex_t on its own
- Add new constructors to Monitor

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@920666 13f79535-47bb-0310-9956-ffa450edef68