Rename feature to: Resend Notification if Down X times consequently

Co-authored-by: Louis Lam <louislam@users.noreply.github.com>

db/patch-monitor-add-resend-interval.sql

@@ -5,6 +5,6 @@ ALTER TABLE monitor
     ADD resend_interval INTEGER default 0 not null;
 
 ALTER TABLE heartbeat
-    ADD last_notified_time DATETIME default null;
+    ADD down_count INTEGER default 0 not null;
 
 COMMIT;