fix: hold mutex when signaling cond var

[elupus]

If mutex is not held when signalling the condition variable, a thread woken up by the mutex unlock,
may have lower priority than the thread waiting
on the condition leading to priority inversion.

We also don't need to wake up waiters when we
clear events, since this will never trigger
return from the wait.

[CLAassistant]

All committers have signed the CLA.

[hefloryd]

Merged, thanks