Improve page split in rtree emulation. Now if splitted result has

Improve page split in rtree emulation. Now if splitted result has
big misalignement, then it tries to split page basing on distribution
of boxe's centers.

Per report from  Dolafi, Tom <dolafit@janelia.hhmi.org>

 Backpatch is needed, change doesn't affect on-disk storage.