Merge branch 'ps/odb-sources' into next

Merge branch 'ps/odb-sources' into next

The object source API is getting restructured to allow plugging new
backends.

* ps/odb-sources:
  odb/source: make `begin_transaction()` function pluggable
  odb/source: make `write_alternate()` function pluggable
  odb/source: make `read_alternates()` function pluggable
  odb/source: make `write_object_stream()` function pluggable
  odb/source: make `write_object()` function pluggable
  odb/source: make `freshen_object()` function pluggable
  odb/source: make `for_each_object()` function pluggable
  odb/source: make `read_object_stream()` function pluggable
  odb/source: make `read_object_info()` function pluggable
  odb/source: make `close()` function pluggable
  odb/source: make `reprepare()` function pluggable
  odb/source: make `free()` function pluggable
  odb/source: introduce source type for robustness
  odb: move reparenting logic into respective subsystems
  odb: embed base source in the "files" backend
  odb: introduce "files" source
  odb: split `struct odb_source` into separate header