Switch to using high resolution timers

[dktapps]

Recently it was discovered that system time changes during SleeperHandler->sleepUntil() will cause the sleeper to break. Changing the time forward causes the sleeper not to sleep at all for that call (which is OK), but changing the time backwards causes an extended sleep until the originally specified time, which has now moved into the future.

Example:

• caller sleeps for now + 50,000 microseconds
• system time changes back by 3 hours
• sleeper handler now sleeps for 3 hours + 50,000 microseconds

This is undesirable for PocketMine-MP for obvious reasons.

Solutions:

• use the HRTime extension (poorly documented, no git source code)
• force an upgrade to PHP 7.3 to get access to the new hrtime() function

[dktapps]

I've attempted to implement this and arrived at the conclusion that providing an API like Threaded->wait(timeout) on SleeperHandler would be better for this purpose. This would shift the responsibility of deciding what kind of time measurement to use to the implementation, and Snooze would then not need to mess around calculating remaining sleep times.
This would make the API slightly more difficult to use, but since only PM uses this library anyway, I don't think that's a particularly big problem.