phabricator/lister: Prevent erroneous scheduler tasks disabling

Previously, the Phabricator lister was disabling some loading tasks while it was not
supposed to. More precisely, due to an invalid index provided to a database query,
the latest created scheduler task was disabled each time a new page of results was
provided to the lister by the Phabricator API. Moreover, database queries were not
filtered according to the Phabricator instance resulting in possible disabling of
scheduler tasks from other instances.

Closes T2000