Wrap slurm_cond_signal() calls with mutexes where needed.

Bug 5231

Change slurmctld/controller.c's purge_thread_lock from static to
global extern, so the mutex can be used in slurmctld/job_mgr.c

slurm_cond_signal() needs to always be wrapped in the same mutex as
slurm_cond_[timed]wait(), or else there is a possibility that
slurm_cond_signal() will trigger before slurm_cond_[timed]wait()
is even listening, most likely causing a deadlock.