hierarchies: Look into exposing rxjs types in FilteringHierarchyDefinition

[grigasp]

At the moment FilteringHierarchyDefinition implements HierarchyDefinition, which is a public type and, because of that, we don't want it to expose rxjs types. As a result, FilteringHierarchyDefinition has to convert all rxjs stuff to promises, which make it less efficient.

Maybe we could do this:

• create an rxjs-aware hierarchy definition,
• make imodel hierarchy provider use the rxjs one,
• have an internal wrapper to map public hierarchy definition into the rxjs one,
• change FilteringHierarchyDefinition to implement rxjs definition rather than the public one.

Need to see if that helps performance / code cleanliness / something else.