Improve behavior of TJoinHelper given the following limitation

of the pthread conditions:

      // When using condition variables there is always a Boolean predicate
      // involving shared variables associated with each condition wait that
      // is true if the thread should proceed. **Spurious** wakeups from the
      // pthread_cond_timedwait() or pthread_cond_wait() functions may occur.
      // Since the return from pthread_cond_timedwait() or pthread_cond_wait()
      // does not imply anything about the value of this predicate, the
      // predicate should be re-evaluated upon such return.


git-svn-id: http://root.cern.ch/svn/root/trunk@39640 27541ba8-7e3a-0410-8455-c3a389f83636