PollInterruptible: Hash access is not threadsafe in Rubinius, even with keys known to be unique.
Use a Queue instead, because the order of interruptions is not important as long as
interruptions blocking for a return value get the right return value.

--HG--
extra : amend_source : 9a6f33de1e0d085a02640ec123e85732bdd61aa2
Use unique endpoints instead of unique contexts in poll_shared specs

--HG--
extra : amend_source : c7fd0d1028bdf9026540711e6fb30ca122cf5fb8