Add an OS mutex to Halide runtime.
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.)


Former-commit-id: cf5298c90763481e80a748c1b10453c482fdd7a8
darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide