Re: suppressing useless wakeups in logical/worker.c

I wrote:
>>> It'd probably be reasonable to file down that sharp edge by instead
>>> specifying that TimestampDifferenceMilliseconds will clamp overflowing
>>> differences to LONG_MAX. Maybe there should be a clamp on the underflow
>>> side too ... but should it be to LONG_MIN or to zero?

After looking closer, I see that TimestampDifferenceMilliseconds
already explicitly states that its output is intended for WaitLatch
and friends, which makes it perfectly sane for it to clamp the result
to [0, INT_MAX] rather than depending on the caller to not pass
out-of-range values.

I checked existing callers, and found one place in basebackup_copy.c
that had not read the memo about TimestampDifferenceMilliseconds
never returning a negative value, and another in postmaster.c that
had not read the memo about Min() and Max() being macros. There
are none that are unhappy about clamping to INT_MAX, and at least
one that was already assuming it could just cast the result to int.

Hence, I propose the attached. I haven't gone as far as to change
the result type from long to int; that seems like a lot of code
churn (if we are to update WaitLatch and all callers to match)
and it won't really buy anything semantically.

regards, tom lane