Core: lazy composite value laading (#339, #411).

Before an update operation is performed on an object its composite value is loaded lazily. Applies to facet updating as well.
Pending for a "get child topic" operation.
See ticket 411.

Update optimization: updating a "composition one" child topic involves no further DB read operation.
Pending for:
    - composition many
    - aggregation one
    - aggregation many
See ticket 339.