Continually locking a mutex m can lead to starvation if all other threads are on the waitq of m.
See https://bugs.ruby-lang.org/issues/21840 for more details.
Solution:
When a thread `T1` wakes up `T2` during mutex unlock but `T1` or any other thread successfully acquires it
before `T2`, then we record the `running_time` of the thread during mutex acquisition. Then during unlock, if
that thread's running_time is less than the saved running time, we set it back to the saved time.
Fixes [Bug #21840]
Luke Gruber
994257ab06