Most notable changes:
- better update concurrency: LUCENE-7868
- TopDocs.totalHits is now a long: LUCENE-7872
- QueryBuilder does not remove the boolean query around multi-term synonyms:
LUCENE-7878
- removal of Fields: LUCENE-7500
For the `TopDocs.totalHits` change, this PR relies on the fact that the encoding
of vInts and vLongs are compatible: you can write and read with any of them as
long as the value can be represented by a positive int.
Adrien Grand