So cdb.DB can be used from the scheduler too

The session ev was nil => panic on Notify.

Also fix the table_modified insert or update sql request: bad column
count vs value count, avoid formatting bogus sql when no table was
changed.

* Add scheduler.Task DBX() and DB() to setup a cdb.DB with and without
  transaction.

* Add cdb.DB Rollback() and Commit()

* Change the trim task config layout:

	scheduler:
	    pprof:
		addr: 127.0.0.1:9998
		enable: false
	    metrics:
		addr: 127.0.0.1:2222
		enable: false
	    task:
		trim:
		  period: 10m
		  batch_size: 500
		  retention: 365
		  table:
		      resmon_log:
			  batch_size: 600
			  retention: 1000
		      svcactions:
			  retention: 350

* Add a timeout field to scheduler.Task

* Allocate a context.WithTimeout(task.timeout)  from scheduler.Task.Exec(),
  so the task doesn't have to do it in most cases.