ForestDB queries wrong key range for descending=true and prefixMatch

[snej]

Spun off from #889 — as exposed by a CBLQueryBuilder test case, there's a bug in the view query support for ForestDB. When descending=true and there's a prefixMatch > 0, the endKey gets modified when it should be the startKey. (Because prefix matches alter the max key.)

[snej]

I checked the corresponding code for all-docs queries (-[CBL_ForestDBStorage getAllDocs:status:]), but it's already doing the correct thing.

There should probably be some common accessors on CBLQueryOptions for minKey and maxKey that take into account descending and prefixMatch. That would move the logic to a single place and make sure everyone does it right.