libfuse: fuse_session_loop_mt() shouldn't pause when exiting worker threads

In fuse_session_loop_mt() don't pause when exiting the worker threads.
The pause() was added in 2.2.1 to prevent segfault on pthread_cancel()
on an exited, detached thread.  Now worker threads are not detached
and pthread_cancel() should work fine even after the thread exited.
Reported by Boris Protopopov
libfuse: fix thread cancel race

Exiting a worker my race with cancelling that same worker.  This caused a
segmenation fault.

Reported and tested by Anatol Pomozov