Add dedicated Index class for unique, non-sparse String fields

[justlep]

The Index class internally uses AVLTree even for unique, non-sparse indexed fields like _id.
Index performance could (most likely) be improved by using a native Map instead of AVLTree for such fields.

TODO

• add a UniqueStringIndex class extending or implementing the existing Index class
• override all relevant methods to use a native Map internally
• make UniqueStringIndex.getBetweenBounds() throw (irrelevant usecase)
• make Datastore use UniqueStringIndex for the _id field by default
• find a way to enable a developer to use UniqueStringIndex for other string indexes as well
• compare benchmarks before/after