Ensure there is no concurrent poller for unary->multipoll

The race here is:
Initially unary poller has one FD, FD_1
Thread 1 adds FD_2, promoting to multipoll, entailing epoll_ctl on FD_1
and FD_2.
Concurrently, Thread 2 returns from unary poll and executes one of
FD_1's callbacks, which ultimately leads to a close(FD_1) which races
with the epoll_ctl by thread 1.

The solution is to ensure that we don't concurrently poll and promote,
which requires a bit of extra care.

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