Full support for timeouts

[mirromutth]

For now, futex and sync (with atomic-wait) implementation does not have support for waiting with a timeout.

macOS does have the __ulock_wait syscall that do support a timeout, but it's a private/unstable API

Usage of private APIs can result in software being rejected from Apple's app stores

See also m-ou-se/atomic-wait#4

There have two solutions:

• Waits for Apple to expose a public API for futex/atomic-wait
• Implements it as a global pool containing mutexes and condition variables

The last option sounds like the only option I can to do something. But, the most common use case for latch is waiting without timeout. The last option will significantly increase the code size for a rare case.