Added basic structures and code to put peers blacklist.
1) when peerA has to remove peerB from its neighborood, it can
put peerB in its local black list if peerB has not been a "good" neighbor.
2) If peerB got blacklisted from peerA, peerA will not insert peerB in its neighborood
until a timeout expires.
3) Blacklist is periodically iterated to look for entries (peers) whose timeout
4) Perceived "goodness" of peerB for peerA can be defined in topology.c. Notice
that this measure should be experience-based (better not blacklist peers
simply looking at their external characteristics [RTT, bandwidth, etc.]).
Signed-off-by: Stefano Traverso <email@example.com>