Now that all of the jobs use the component finalization callbacks,
there's no use for the heavy-hammer .exit callback anymore.

job_exit becomes a glorified type shim so that we can call
job_completed from aio_bh_schedule_oneshot.

Move these three functions down into job.c to eliminate a
forward reference.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 20180906130225.5118-12-jsnow@redhat.com
Reviewed-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>