Merge branch 'ps/object-store-midx'

Merge branch 'ps/object-store-midx'

Redefine where the multi-pack-index sits in the object subsystem,
which recently was restructured to allow multiple backends that
support a single object source that belongs to one repository.  A
midx does span mulitple "object sources".

* ps/object-store-midx:
  midx: remove now-unused linked list of multi-pack indices
  packfile: stop using linked MIDX list in `get_all_packs()`
  packfile: stop using linked MIDX list in `find_pack_entry()`
  packfile: refactor `get_multi_pack_index()` to work on sources
  midx: stop using linked list when closing MIDX
  packfile: refactor `prepare_packed_git_one()` to work on sources
  midx: start tracking per object database source