This commit adds the boolean similarity scoring from Lucene to
Elasticsearch. The boolean similarity provides a means to specify that
a field should not be scored with typical full-text ranking algorithms,
but rather just whether the query terms match the document or not.
Boolean similarity scores a query term equal to its query boost only.
Boolean similarity is available as a default similarity option and thus
a field can be specified to have boolean similarity by declaring in its
mapping:
"similarity": "boolean"
Closes#6731
Ali Beyad
Colin Goodheart-Smithe
Clinton Gormley
Adrien Grand
Dongjoon Hyun
Robert Muir
Jim Ferenczi