diversify connections and disconnection based on prefixes of IP addresses

(related to issue #214 (closed))

The main contribution of this MR is a data structure in src/p2p_pool/p2p_points_partition.ml to partition points and/or connections into equivalence classes, and three associated functions:

partition_disconnected: partitions (disconnected) points of a given pool into equivalence classes

partition_connected: partitions connections into equivalence classes

pick: picks (at most) n elements from the given equivalence class.

This code is parameterized by a hash function on IP addresses. Currently, it is instantiated with a function that hashes the prefixes of IP adresses, and the result is used to:

• diversify new established connections when calling too_few_connections (use of partition_disconnected + pick)

• diversify connections when killing connections with too_many_connections (use of partition_connected + pick)

Note that:

• Currently, connections that are in private mode are not killed. But, I'll probably change this to rather keep white-listed connections.

• I also modified a little bit P2p_pool.accept to randomly accept one additional incoming connection so that too_many_connections can be triggered

What do you think about this @hnrgrgr @chambart ?

EDIT: After a discussion with @chambart, I changed the code to "protect" trusted peers from disconnection instead of private nodes.