Missed in #69.
The `TermSet` fast fields implementation cribbed from `RangeQuery`'s fast fields implementation: ... which also has this bug. Will fix upstream.
The `TermSet` `Query` currently produces one `Scorer`/`DocSet` per matched term by scanning the term dictionary and then consuming posting lists. For very large sets of terms and a fast field, it is faster to scan the fast field column while intersecting with a `HashSet` of (encoded) term values.
Following the pattern set by the two execution modes of `RangeQuery`, this PR introduces a variant of `TermSet` which uses fast fields, and then uses it when there are more than 1024 input terms (an arbitrary threshold!).
Performance is significantly improved for large `TermSet`s of primitives.
