Enable pool cache on knote pool

Enable pool cache on knote pool

Use the pool cache to reduce the overhead of memory management in
function kqueue_register().

When EV_ADD is given, kqueue_register() pre-allocates a knote to avoid
potential sleeping in the middle of the critical section that spans
from knote lookup to insertion. However, the pre-allocation is useless
if the lookup finds a matching knote.

The cost of knote allocation will become significant with kqueue-based
poll(2) and select(2) because the frequency of allocation will increase.
Most of the cost appears to come from the locking inside the pool.
The pool cache amortizes it by using CPU-local caches of free knotes
as buffers.

OK dlg@ mpi@