MINOR: clock: split local and global date updates

MINOR: clock: split local and global date updates

Pollers that support busy polling spend a lot of time (and cause
contention) updating the global date when they're looping over themselves
while it serves no purpose: what's needed is only an update on the local
date to know when to stop looping.

This patch splits clock_pudate_date() into a pair of local and global
update functions, so that pollers can be easily improved.