DfsPhase captures terms used for scoring a query in order to build global term statistics across
multiple shards for more accurate scoring. It currently does this by building the query's `Weight`
and calling `extractTerms` on it to collect terms, and then calling `IndexSearcher.termStatistics()`
for each collected term. This duplicates work, however, as the various `Weight` implementations
will already have collected these statistics at construction time.
This commit replaces this round-about way of collecting stats, instead using a delegating
IndexSearcher that collects the term contexts and statistics when `IndexSearcher.termStatistics()`
is called from the Weight.
It also fixes a bug when using rescorers, where a `QueryRescorer` would calculate distributed term
statistics, but ignore field statistics. `Rescorer.extractTerms` has been removed, and replaced with
a new method on `RescoreContext` that returns any queries used by the rescore implementation.
The delegating IndexSearcher then collects term contexts and statistics in the same way described
above for each Query.
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