Call the internal _process_stopped handler

processEnded was calling only the user overridable process_stopped
handler. Now it calls the internal one instead, which will eventually
call the overridable one.
Rework deferred handling for assignments

The stopped and started deferreds are now no longer deferred lists, but
normal deferreds that are fired manually under the right circumstances.
This allows jobtype implementors to start new processes after an earlier
one has already finished.
This also addresses the bug where the stopped deferred never got fired,
which made it impossible, among other things, to cleanly shut down the
agent.
Set self.start_called after the fact
Log if there were failed processes in assignment
Merge pull request #117 from guidow/rework_assignment_deferreds

Rework assignment deferreds