Add an OS mutex to Halide runtime.

Document an issue with the POSIX thread pool.

Ensure memoization cache is cleaned up when a JITted module is
shutdown. (Also provides a way to clear the cache from AOT code.)
Recommit changes form cf5298c90763481e80a748c1b10453c482fdd7a8.
Merge branch 'mutexchanges'

Conflicts:
	src/runtime/scoped_mutex_lock.h