Add PeerFinder peer discovery logic and unit test

src/ripple_data/protocol/ripple.proto

@@ -14,6 +14,7 @@ enum MessageType
     mtCONTACT               = 11;
     mtGET_PEERS             = 12;
     mtPEERS                 = 13;
+	mtANNOUNCE              = 14;
 
     // operations for 'small' nodes
     mtSEARCH_TRANSACTION    = 20;
@@ -228,6 +229,14 @@ message TMPeers
     repeated TMIPv4EndPoint nodes   = 1;
 }
 
+message TMAnnounce
+{
+	required bytes   serverID             = 1; // The ID of this server
+	required bool    privatePeer          = 2; // This peer is private - the announce will not be propagated
+	required uint32  hopCount             = 3; // The hop count of this server
+	required bytes   viaPeerID            = 4; // The ID of the peer through which we know this server
+	repeated TMIPv4EndPoint connectPoints = 5; // Addresses on which this server accepts connections (can be empty)
+};
 
 message TMSearchTransaction
 {