Allow to pass new list of indexed entities in update schema API

Allow to update list of indexed entities in a cache after the was created. That would be useful for the following scenario (taken from Zulip discussion): 

As we use nested entities within root entity a lot sometimes the root entities become too big. Therefore for sake of performance (reading and writing to nested entities always mean loading/writing whole root entity) we needed to move some nested entities to root level so it is manageable on its own. Then, the parent entity only stores a reference to id of the child entities (this way we basically model one-to-many relationship similarly as it is done in db).

To have it all in one place we decided to store child entities in the same cache as parent entities, therefore we need to define this entity in the <indexed-entities> block. The problem is, that later if we decide to split some more entities into parent-child relationship we will need to add he child entity into <indexed-entities>block of existing cache as it is no longer nested entity.

This could be easily worked around by introduction of new cache each time we do similar split, however, we find it nicer to have it all stored in the same cache.

CC: fercoli@redhat.com mhajas@redhat.com