[DimensionMapper] Be less eager to fetch when slicing

[loichuder]

This is a problem that appears when slicing through a dataset with heavy slices.

Say I want to look at the latest slice of the slicing dimension: I need to make the thumb go through many (if not all) slices between the first and the last.

This triggers many slow slice fetches which are not useful as I am only interested in the last one in my example.

We could have some throttling or launch the fetch only when releasing the thumb.