Use either pthread_rwlock_t or std::mutex.

This simplifies util/mutex.h greatly while keeping
reader/writer locks for those who already had them.

Change-Id: Ia17c41db48c67b03ae5789a3adf0d6eaa3567aab
Reviewed-on: https://code-review.googlesource.com/6713
Reviewed-by: Paul Wankadia <[email protected]>
2 files changed