Multiphased grouping execution in backend

[baldersheim]

Currently we execute the full grouping in the back in a single phase if we discover that is necessary. This is necessary if we at any level have ordering on something else than rank.

In the case that you have multiple levels with different ordering this is not optimal. We should instead do multi pass in the backend. We could then optionally reorder hits between the phases if that is.

This should give significant improvement as we do not have to build the full grouping tree, but only the part we are really interested in.