Improved the processing of background task to enable more advanced functionalities like queuing -reintegrated from trunk

git-svn-id: http://svn.code.sf.net/p/itop/code/branches/2.0.3@3301 a333f486-631f-4898-b8df-5754b55c2be0
Improved the processing of background task to enable more advanced functionalities like queuing (factorized the error handling) -reintegrated from trunk

git-svn-id: http://svn.code.sf.net/p/itop/code/branches/2.0.3@3303 a333f486-631f-4898-b8df-5754b55c2be0
#976: make sure that we do not bypass the method that computes the reference for newly created tickets.

git-svn-id: http://svn.code.sf.net/p/itop/code/branches/2.0.3@3308 a333f486-631f-4898-b8df-5754b55c2be0
Protect dashboards against invalid queries in "grouped by" dashlets.

git-svn-id: http://svn.code.sf.net/p/itop/code/branches/2.0.3@3309 a333f486-631f-4898-b8df-5754b55c2be0