Update behaviour contract for ALL keyword

Created by: davecromberge

This PR is the first step in implementing the proposal https://github.com/dalmatinerdb/dalmatinerdb/issues/75

There were three alternatives to encoding this keyword in the callback:

1. Pass the atom undefined in place of a metric
2. Create a new callback function lookup_all
3. Tagging the lookup query tuple as all instead of in.

I have opted for the 1st option in this PR, as it aligns closely to the spec proposed in https://github.com/dalmatinerdb/dqe_idx/issues/7