Merge branch 'osx-clock'

Moves the thread into real-time scheduling class, as recommended in [1], it
fixes the separate test provided by @ygj6 [2] everywhere (github actions,
travis-ci, appveyor) under osx.

  [1]: https://developer.apple.com/library/archive/technotes/tn2169/_index.html
  [2]: https://github.com/azat-archive/osx-timers/commit/dde1a6e4d04506d0e0fb193ebb9f49ae25873be6

Although even after this changes the following time-related tests failed
on travis-ci:
- no_events
- del_wait

But anyway I guess #940 can be closed, since this fixes the issue in common.

* osx-clock:
  test: Use THREAD_* wrappers in del_notify/del_wait
  test: move threads created with THREAD_START() to realtime scheduling class too
  test: put thread into real time scheduling class on osx for better latencies

Closes: #940