change Task status in DB task table from SUBMITTED to COMPLETED or FAILED when DAG completes

[belforte]

basically we want to be able to count running DAG's per user in order to do some task scheduling #7485

hmm.. maybe we can leave Task status alone and e.g. define later how and when exactly to call it FAILED but add a column with Dagman Status ?

In any case this should be one API call in cache_statup.py

Need to be able to also catch FAILED->SUBMITTED transition at crab resubmit time.

[belforte]

FYI @mapellidario , please comment as you see fit