Allow efficiently fetching all values for facetable fields

[mdavis95]

Currently the only way to fetch all values for a field is to use a keyword analyzer and use GetTerms request (which will contained deleted values) on the keyword analyzed field or to do a *:* search with a facet on a facetable field.

The zulia facet index has all the values and they can be fetched without running a *:* query and counting every document.

Add a GetFacetValuesRequest that takes a list of optional categories and optional path. Facet values are never deleted from the taxonomy so this has issues like GetTerms