Enforce isc_work enqueue loop affinity

Enforce isc_work enqueue loop affinity

Add a REQUIRE(isc_loop() == loop) assertion to isc_work_enqueue()
to strictly enforce that work is enqueued from the loop it is
assigned to. This loudly prohibits cross-thread queue manipulation
before it inevitably turns into a concurrency debugging nightmare.

(cherry picked from commit f1311d2d196d381b2170c4b3d54262874a5d424e)