Add bulk scorer for sparse-index-based scorers

Our queries that take advantage of sparse indexes can match many documents at once. We shoud create `BulkScorer`s for them, to be able to use the `Collector#collect(DocIdStream)` API and collect several doc IDs at once.

Start by locating the sparse-index-based scorers and the Collector#collect(DocIdStream) API mentioned in the issue. Trace how those scorers currently collect matching documents; done means they provide BulkScorers that can collect several matching document IDs at once.

Written by the indexing model from the issue text.