Fix busy loop in ae.c when timer event is about to fire (#8764)

The code used to decide on the next time to wake on a timer with
microsecond accuracy, but when deciding to go to sleep it used
milliseconds accuracy (with truncation), this means that it would wake
up too early, see that there's no timer to process, and go to sleep
again for 0ms again and again until the right microsecond arrived.

i.e. a timer for 100ms, would sleep for 99ms, but then do a busy loop
through the kernel in the last millisecond, triggering many calls to
beforeSleep.

The fix is to change all the logic in ae.c to work with microseconds,
which is good since most of the ae backends support micro (or even nano)
seconds. however the epoll backend, doesn't support micro, so to avoid
this problem it needs to round upwards, rather than truncate.

Issue created by the monotonic timer PR #7644 (redis 6.2)
Before that, all the timers in ae.c were in milliseconds (using
mstime), so when it requested the backend to sleep till the next timer
event, it would have worked ok.