Add deadlock counter to pg_stat_database

Adds a counter that tracks number of deadlocks that occurred in
each database to pg_stat_database.

Magnus Hagander, reviewed by Jaime Casanova

src/backend/storage/lmgr/deadlock.c

@@ -938,6 +938,8 @@ DeadLockReport(void)
 					  pgstat_get_backend_current_activity(info->pid, false));
 	}
 
+	pgstat_report_deadlock();
+
 	ereport(ERROR,
 			(errcode(ERRCODE_T_R_DEADLOCK_DETECTED),
 			 errmsg("deadlock detected"),