Further adjustment to random() seed initialization.

Per complaint from Tom Lane, don't chomp the timestamp at 32 bits, so we
can shift in some of its higher bits.

Discussion: https://postgr.es/m/14712.1542253115%40sss.pgh.pa.us

src/backend/postmaster/postmaster.c

@@ -2532,9 +2532,9 @@ InitProcessGlobals(void)
 	 * in a given time period.  Since that would leave only 20 bits of the
 	 * timestamp that cycle every ~1 second, also mix in some higher bits.
 	 */
-	srandom(((unsigned int) MyProcPid) ^
-			((unsigned int) MyStartTimestamp << 12) ^
-			((unsigned int) MyStartTimestamp >> 20));
+	srandom(((uint64) MyProcPid) ^
+			((uint64) MyStartTimestamp << 12) ^
+			((uint64) MyStartTimestamp >> 20));
 }