Add debugging code for the "latch already owned" error.

We've seen such a case on a stable release but it is hard to debug via the
message only, so let's provide more details in the error message.

src/backend/storage/ipc/latch.c

@@ -260,7 +260,8 @@ OwnLatch(volatile Latch *latch)
 #endif
 
 	if (latch->owner_pid != 0)
-		elog(ERROR, "latch already owned");
+		elog(ERROR, "latch already owned by pid %d (is_set: %d)",
+			 latch->owner_pid, (int) latch->is_set);
 
 	latch->owner_pid = MyProcPid;
 }