split housekeeping jobs to separate scheduled tasks on longer cooldown intervals, add table to record task execution timestamps, bump schema

sql/pgsql/migrations/150.sql

@@ -0,0 +1,5 @@
+create table ttrss_scheduled_tasks(
+   id serial not null primary key,
+   task_name varchar(250) unique not null,
+   last_rc integer not null,
+   last_run timestamp not null default NOW());