P2P: Periodically eject unreachable peers from AddressBook + Replace Peer list IndexMap by BucketMap

[SyntheticBird45]

What

Fixes parts of: #178

• Implements PeerPinger in cuprate-p2p. This background task will periodically fetch a random white and gray peers and try to ping them. Replace them in the list if successful, drop it otherwise.
• Added a new AddWhitePeer request to AddressBookRequest. Needed for the PeerPinger to replace white peers.
• Renamed Bucket as BucketMap.
• Completed BucketMap API for usage within peer_list.rs in cuprate-address-book.
• Added Bucketable trait as a dependency for NetZoneAddress and implemented discrimination to SocketAddr.

Why

As described in #178, Cuprate will at the moment keep indefinitely any peer contained in its list, even ones that may not be reachable anymore in the long/mid-term.

The use of BucketMap instead of IndexMap will ensure that we store at most N amount of IP addresses from the same subnet. This ensure a form of diversity and could potentially help at mitigating monero-project/research-lab#126

[SyntheticBird45]

@Boog900 I would appreciate an ahead of time review of the API changes before I pass to test coverage and documentation fixes.
Also I defined N at 8. So the bucket is storing up to 8 IP addresses of the same subnet. Do you wanna change it?