We schedule new harvests based on harvest definitions that are not yet finished with the previous harvest

The scheduler schedules harvests based on harvest definitions without consulting
information about previous harvests at all.
Thus the same harvest may be scheduled again even if an old harvest from the
same harvest definition is not yet finished. This may end up with congestion in
the harvester queues.
Worse still snapshot harvests based on a previous harvest definition is
scheduled even if the previous harvest definition is not yet done! The result of
this is that only the domains that have been harvested so far by the previous
harvest definition are included in the next phase.
It can be checked whether previous harvests are done by checking open jobs made
from this harvest definition for statuses other than FAILED or DONE. Any such
jobs mean that the harvest is not yet done.
There is however a risk that some message is lost along the way and a job is
never updated so at least some sort of warning must be issued if a scheduling
is delayed due to unfinished harvests.
NOTE: This bug is originally from Bugzilla bug_id=318.
This bug was previously assigned to Unassigned.