Log a message when resorting to SIGKILL during shutdown/crash recovery.

Currently, no useful trace is left in the logs when the postmaster
is forced to use SIGKILL to shut down children that failed to respond
to SIGQUIT.  Some questions were raised about how often that scenario
happens in the buildfarm, so let's add a LOG-level message showing
that it happened.

Discussion: https://postgr.es/m/1850884.1599601164@sss.pgh.pa.us

src/backend/postmaster/postmaster.c

@@ -1850,6 +1850,8 @@ ServerLoop(void)
 			(now - AbortStartTime) >= SIGKILL_CHILDREN_AFTER_SECS)
 		{
 			/* We were gentle with them before. Not anymore */
+			ereport(LOG,
+					(errmsg("issuing SIGKILL to recalcitrant children")));
 			TerminateChildren(SIGKILL);
 			/* reset flag so we don't SIGKILL again */
 			AbortStartTime = 0;