Heads up: slow downs with luceneutil (including nightly benchmarks)

Hello all,

For reference, I just merged this PR[0] to luceneutil, which "pollutes"
call sites of DocIdSetIterator#nextDoc, DocIdSetIterator#advance, Bits#get
(live docs), SimScorer#score and others to better reflect what performance
may look like on a production system that handles diverse workloads.

A consequence is that luceneutil now report worse performance. In
particular, nightly benchmarks are expected to get noticeable drops on many
tasks. This behavior in enabled by default but it is possible to disable it
by passing pollute=False when initializing a Competitor.

[0] https://github.com/mikemccand/luceneutil/pull/436

-- 
Adrien