TT#182702 Index "inserted_time"+"status" in watchers

New index for watchers table for columns "inserted_time" and "status".
This is a little performance boost since the function
ps_watchers_db_timer_clean() cleans pending subscriptions
using this columns inside the "where" clause.

Change-Id: I9cb2220f6fe89aa2070d9a739c4a3a0417e3de71

db_scripts/diff/15724.down

@@ -0,0 +1,3 @@
+USE kamailio;
+
+DROP INDEX inserted_time_status_idx ON watchers;

db_scripts/diff/15724.up

@@ -0,0 +1,3 @@
+USE kamailio;
+
+CREATE INDEX inserted_time_status_idx ON watchers (`inserted_time`, `status`);