fm: map[path]value.

rfm: map[rpath]value.

Search: get id group with (rpath, val) conditions.

Search: one object with prefix `id:`.

Search: objects with prefix `id:`.

init global 'DbGrp', if dir is empty, and no dbs, use memory mode.

[ByRPMap's Key-Type, Object-Type].

[Object-Type].

[Object-Type].

update or insert an object.

update or insert objects.

global, for keeping single instance.