Associate index scrubbing with index maintainers

[jjezra]

This is a preparation for new types of index scrubbing tools and methods.

[ScottDugas]

This seems like a better separation of concerns. Right now the index scrubbing is tightly coupled to standard value indexes, making it unusable for other index types. Pushing some of the logic down into classes provided by the index maintainer would allow scrubbing of more index types.