I was looking at some queries where Lucene performs significantly worse than
Tantivy at https://tantivy-search.github.io/bench/, and found out that we get
quite some overhead from implementing `BooleanScorer` on top of `BulkScorer`
(effectively implemented by `DefaultBulkScorer` since it only runs term queries
as boolean clauses) rather than `Scorer` directly.

The `CountOrHighHigh` and `CountOrHighMed` tasks are a bit noisy on my machine,
so I did 3 runs on wikibigall, and all of them had speedups for these two
tasks, often with a very low p-value.

In theory, this change could make things slower when the inner query has a
specialized bulk scorer, such as `MatchAllDocsQuery` or a conjunction. It does
feel right to optimize for term queries though.