Improve Vicinity algorithm

[NicolasDP]

@vincenthz is right here.

The available are in a BTreeSet for it makes it easier for the ring module, however it is not efficient at all for the case of the vicinity.

Here we need to search for something more in depth. Maybe have the vicinity module keep an indexing of the available nodes to allow a fast selection of the closest nodes by topic.

Originally posted by @NicolasDP in #9 #9 (comment)