ddfs_tag: avoid doing delete_op on the +deleted tag.

If something happens to this tag and it becomes missing, we still should not try
to do a delete_op on it because it is going to time out.
event_server: increase the total number of tasks whenever a task gets ready.

In the new model of task dispatching, we no longer know the number of tasks in
advance. Therefore, we have to increase this number every time a new job is
available.
disco_web: do not count the pending tasks among the total tasks of a stage.

The pending tasks are in the job_coordinator and have not been given to the
disco_server. Therefore, they should not be counted when calculating the number
of waiting tasks.
Merge pull request #545 from pooya/develop

Develop