Revert "optimize when disjunction query has only a single child"
Revert "optimize when disjunction query has only a single child"

See also: https://issues.couchbase.com/browse/MB-21046

This reverts commit 6d6fae289509c9deb27710e6f28d56b1ae587c06.

It turns out that boolean searcher was depending on its 'should'
constituent (a disjunction query) and its min state changes, so a
rewrite wasn't safe/correct given this situation.

steveyen/bleve,steveyen/bleve,blevesearch/bleve,slavikm/bleve,blevesearch/bleve,slavikm/bleve