Merge branch 'ps/odb-sources' into ps/object-counting

* 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