Make ConditionVariable#wait return false if timed out

[jeremyevans]

This was the historical Ruby 1.8 behavior.

This uses @nobu's patch from #2884, but restores backwards compatibility for Mutex#sleep to always return the slept time. It adds Mutex#sleep_for that requires a timeout argument and will return nil if the sleep times out.

[nobu]

Could you add the ticket to DevelopersMeeting20210317Japan?

[jeremyevans]

At the dev meeting, matz approved modifying Mutex#sleep, so I'm removing the second patch. Will merge if CI passes.

[ioquatix]

LGTM thanks for making the interface more communicative.