add indexes, improve queries

db/patch11.sql

@@ -0,0 +1,10 @@
+-- You should not modify if this have pushed to Github, unless it does serious wrong with the db.
+BEGIN TRANSACTION;
+
+-- For sendHeartbeatList
+CREATE INDEX monitor_time_index ON heartbeat (monitor_id, time);
+
+-- For sendImportantHeartbeatList
+CREATE INDEX monitor_important_time_index ON heartbeat (monitor_id, important,time);
+
+COMMIT;