[FTS] cbft needs "best effort" option for query requests (partial results)

During a query of an index or an index alias, there should be an optional query control parameter that allows the client to ask for "best effort".

That is, if an node is down or a pindex is unavailable, to give back partial results.

An application might be interested in this in the theory that showing any results, even imperfect results, is better in some situations than showing an error page to the user.

See also: https://github.com/couchbase/cbft/issues/1