Optional sorting of predicates

Created by: aprokop

Fix #206 (closed).

I added a new boolean parameter. I opted to have it before buffer_size as it is common from both kNN and spatial searches, while the parameters after it are specific (buffer_size to spatial, which to kNN). This choice made the patch slightly larger than it could have been.

I also decided to keep the default to sorted as of now. I'm not married to it, but this allows us to still be able to compare benchmark results with older runs.