Pass defer_commit in context on get_site_user calls

See ckan/ckan#1714. Until that is fixed properly, the `defer_commit`
flag avoids some `DetachedInstanceErrors` happening during the
harvesting.
[#70] Fix deletion of harvested CSW records

The object id was pushed to the list returned by gather_stage before
being saved on the db, so None was added, causing an exception in the
Redis queue