BIP155: use a dedicated message for signalling

Change signaling of support for the new `addrv2` messages to be done via
a dedicated message `sendaddrv2` instead of protocol bump.

The drawback of using a protocol bump is that the protocol version is
not a bitmask and if a node wants to announce support for `addrv2` this
would imply support for all prior features included in that protocol
version.

Author: vasild

bip-0155.mediawiki

@@ -130,14 +130,13 @@ Clients SHOULD reject messages that contain addresses that have a different leng
 
 See the appendices for the address encodings to be used for the various networks.
 
-==Compatibility==
-
-Send <code>addrv2</code> messages only, and exclusively, when the peer has a certain protocol version (or higher):
-<source lang="c++">
-//! gossiping using `addrv2` messages starts with this version
-static const int GOSSIP_ADDRV2_VERSION = 70016;
-</source>
-For older peers keep sending the legacy <code>addr</code> message, ignoring addresses with the newly introduced address types.
+==Signaling support and compatibility==
+
+Introduce a new message type <code>sendaddrv2</code>. Sending such a message indicates that a node can understand and prefers to receive <code>addrv2</code> messages instead of <code>addr</code> messages. I.e. "Send me addrv2".
+
+<code>sendaddrv2</code> SHOULD be sent after receiving the <code>verack</code> message from the peer.
+
+For older peers, that did not emit <code>sendaddrv2</code>, keep sending the legacy <code>addr</code> message, ignoring addresses with the newly introduced address types.
 
 ==Reference implementation==