Prepare subnet selection speedup: patrica trees

Patricia trees are the optimal general data structure for IP prefix lookup: the number of comparisons is bounded by the number of bits so 32 or 128 for IP, the real number depends on the number of nodes where children differs and in the real world is far lower.

BTW the current code just iterates on all subnetworks so is linear in the number of subnets (average case N/2, N for the worst case which includes the not found case). The patricia tree gives also overlaps (equality or inclusion).