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: 67b04dc5ae26ee489f051e7ead819f482d2fccb6
darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide