Core: Change the default cache filter impl from FixedBitSet to WAH8DocIdSet.

A few months ago, Lucene switched from FixedBitSet to WAH8DocIdSet in order
to cache filters. WAH8DocIdSet is especially better when dealing with sparse
sets: iteration is faster, memory usage is lower and there is an index that
helps keep advance fast.

This doesn't break existing code since #6280 already made sure that there was no
abusive cast from DocIdSet to Bits or FixedBitSet and #7037 moved consumers of
the filter cache that absolutely need to get fixed bitsets to their own cache.

Since cached filters will be more memory-efficient, the filter cache size has
been decreased from 10 to 5%. Although smaller, this might still allow to cache
more filters.