Add value as parameter for knn distance callbacks

[tzaeschke]

Add value as parameter for kNN distance callbacks. Distance calculations for knn queries should have access to the "value", e.g. to exclude certain entries from the kNN result or to add a value's property to the distance..

[tzaeschke]

This doesn't work. Completely filtering out entries and nodes is already possible via the FILTER. Returning a different distance depending on the entry's value does not work because the parent nodes do not know about their entries' values and may give wrong ranking.

In theory it probably work if the variation in distance is always increasing the distance, i.e. the distance of any entries remains always larger than the distance of the parent nodes. This should work because we are only checking the nodes' minimum distance. However it would be:

• brittle (because we cannot efficiently ensure that the entry's distance is always larger)
• inefficient (because the parent nodes are less accurate in giving a ranking of their entries)

Rejected.