Bundle validation: verify with charms.

Validate the bundle with respect to charms in the store.
I don't think it's necessary to open the blob. All the information is available in the
Entity document. I'd define a type, say entityCharm, which implements the
methods required by the charm.Charm interface. 

e.g.
    type entityCharm mongodoc.Entity
    func (e *entityCharm) Meta() *charm.Meta {
        return e.CharmMeta
    }
    ... etc

Then you can just retrieve all the entities from the entities collection with the
appropriate ids (entities.Find(bson.D{{"_id", bson.D{{"$in", ids}}}}).All(&found)
might work ok) put them into a map and return them as is.