BlockTree is our default terms dictionary today, but it doesn't
support term ords, which is an optional API in the postings format to
retrieve the ordinal for the currently seek'd term, and also later
seek by that ordinal e.g. to lookup the term.
This can possibly be useful for e.g. faceting, and maybe at some point
we can share the postings terms dict with the one used by sorted/set
DV for cases when app wants to invert and facet on a given field.
The older (3.x) block terms dict can easily support ords, and we have
a Lucene41OrdsPF in test-framework, but it's not as fast / compact as
block-tree, and doesn't (can't easily) implement an optimized
intersect, but it could be for fields we'd want to facet on, these
tradeoffs don't matter. It's nice to have options...