LimitFields optimization for hit-terms

[dtspence]

Implements the following optimizations for limit fields:

• Key/group parsing: parse the key/group in reduced passes.
• Hit-term pre-evaluation: evaluate hit-terms up front and cache for use later.

The hit-term pre-evaluation and key/group parsing have the most benefit throughput-wise. There was another optimization tested around the per-field cache lookups, but the improvement may not be noticable per-query and requires additional query testing to determine impact.