Improve TimestampDifferenceMilliseconds to cope with overflow sanely.

We'd like to use TimestampDifferenceMilliseconds with the stop_time
possibly being TIMESTAMP_INFINITY, but up to now it's disclaimed
responsibility for overflow cases.  Define it to clamp its output to
the range [0, INT_MAX], handling overflow correctly.  (INT_MAX rather
than LONG_MAX seems appropriate, because the function is already
described as being intended for calculating wait times for WaitLatch
et al, and that infrastructure only handles waits up to INT_MAX.
Also, this choice gets rid of cross-platform behavioral differences.)

Having done that, we can replace some ad-hoc code in walreceiver.c
with a simple call to TimestampDifferenceMilliseconds.

While at it, fix some buglets in existing callers of
TimestampDifferenceMilliseconds: basebackup_copy.c had not read the
memo about TimestampDifferenceMilliseconds never returning a negative
value, and postmaster.c had not read the memo about Min() and Max()
being macros with multiple-evaluation hazards.  Neither of these
quite seem worth back-patching.

Patch by me; thanks to Nathan Bossart for review.

Discussion: https://postgr.es/m/3126727.1674759248@sss.pgh.pa.us

src/backend/replication/walreceiver.c

@@ -445,7 +445,7 @@ WalReceiverMain(void)
 				pgsocket	wait_fd = PGINVALID_SOCKET;
 				int			rc;
 				TimestampTz nextWakeup;
-				int			nap;
+				long		nap;
 
 				/*
 				 * Exit walreceiver if we're not in recovery. This should not
@@ -528,15 +528,9 @@ WalReceiverMain(void)
 				for (int i = 0; i < NUM_WALRCV_WAKEUPS; ++i)
 					nextWakeup = Min(wakeup[i], nextWakeup);
 
-				/*
-				 * Calculate the nap time.  WaitLatchOrSocket() doesn't accept
-				 * timeouts longer than INT_MAX milliseconds, so we limit the
-				 * result accordingly.  Also, we round up to the next
-				 * millisecond to avoid waking up too early and spinning until
-				 * one of the wakeup times.
-				 */
+				/* Calculate the nap time, clamping as necessary. */
 				now = GetCurrentTimestamp();
-				nap = (int) Min(INT_MAX, Max(0, (nextWakeup - now + 999) / 1000));
+				nap = TimestampDifferenceMilliseconds(now, nextWakeup);
 
 				/*
 				 * Ideally we would reuse a WaitEventSet object repeatedly