Doing a unique mongo query for each unit could add a lot of overhead. We should query for lots of units at once. See ``pulp_rpm.plugins.importers.yum.utils.paginate``.
Here's some approximate pseudo-code of what I have in mind:

```
for units_page in paginate(units_to_download):
  criteria = # make one criteria that has all the unit keys for the page
  result = # do the search
  for unit in units_page:
    if unit in result:  # I'm sure this won't work as-is, but you get the idea
      unit = sync_conduit.init_unit(...)
      sync_conduit.save_unit(unit)
    else:
      yield unit