Fix perf regression in the benchmark for the Count collector.
Fix perf regression in the benchmark for the Count collector.

In order to reduce IO, we introduced a way to instanciate a dummy
constant FieldnormReader which worked by allocating a buffer with
as many bytes as there are docs in the segments.

This allocation is not a negligible by any mean.

This PR works by offering two implementation for the
FieldnormReader.
The const field norm reader simply returns the same value all of the
time, while the array based one does the same as the current one.

tantivy-search/tantivy,tantivy-search/tantivy