The last_modified field is intended to support setting the
orig-last-modified field in the S3 backend when importing, basically
to keep track of this bit of pre-export data for debugging. In the
event that it isn't available, the correct thing to do is not write
out an invalid `last_modified` field; we should just not write it out
at all.
We now instantiate NEXT_ID in sequencer.py, which avoids
having multiple modules make multiple copies of a sequencer
and possibly causing id collisions.
Steve Howell
Tim Abbott