More CBLQueryPlanner improvements

* Added convenience init that takes predicate as an NSString.
* Added -explanation method for troubleshooting & learning.
* Handle _queryKeys expression evaluating to an NSSet or NSDictionary.
Added postFilter support to all-docs queries.
all-docs query optimizations

* Don't look up attachments if rev's no_attachments column is true
* Get document contents if there's a filter, because it's almost certain to look at them.