- fix assertion in add_dedupe_request()
- ctxt->num_queued in dedupe.c wasn't being updated along with the queued
  list in a couple places, fix that.
- Putting filerecs on lists for the dedupe context doesn't work out because a
  filerec can be added to a context multiple times.
  - instead allocate a request object for this
  - it turns out the code looks cleaner this way