WL#493: Make RTree Keys Work On Non-Spatial Fields

Affects: Server-7.1   —   Status: Assigned   —   Priority: Medium

We have RTree keys implementation, but now it works only with spatial
objects an is used only with special spatial functions like "WITHIN".

RTree keys are slower, than BTree, but they are "symmetric" about keyparts. So 
one key could serve wide range of selects.