MINOR: pollers: only update the local date during busy polling

MINOR: pollers: only update the local date during busy polling

This patch modifies epoll, kqueue and evports (the 3 pollers that support
busy polling) to only update the local date in the inner polling loop,
the global one being done when leaving the loop. Testing with epoll on
a 24c/48t machine showed a boost from 53M to 352M loops/s, indicating
that the loop was spending 85% of its time updating the global date or
causing side effects (which was confirmed with perf top showing 67% in
clock_update_global_date() alone).