Add anchor connections

Based on https://eprint.iacr.org/2015/263.pdf 4. Anchor connections.

Peer list serialisation version bumped to 5.

src/cryptonote_config.h

@@ -107,6 +107,7 @@
 #define P2P_DEFAULT_INVOKE_TIMEOUT                      60*2*1000  //2 minutes
 #define P2P_DEFAULT_HANDSHAKE_INVOKE_TIMEOUT            5000       //5 seconds
 #define P2P_DEFAULT_WHITELIST_CONNECTIONS_PERCENT       70
+#define P2P_DEFAULT_ANCHOR_CONNECTIONS_COUNT            2
 
 #define P2P_FAILED_ADDR_FORGET_SECONDS                  (60*60)     //1 hour
 #define P2P_IP_BLOCKTIME                                (60*60*24)  //24 hour