BZ-1105122 - Avoid redundant calls of UserGroupCallback methods

(cherry picked from commit 096da268dbf2a49d5b9aa713cb8a1547533faa3e)
BZ-1154576 - TaskService.getTasksAssignedAsPotentialOwners() returns excluded owners too

(cherry picked from commit 712cb74e4c95927dde84ccaa41805710b7d030f2)