net: advertise support for ADDRv2 via new message

Introduce a new message `sendaddrv2` to signal support for ADDRv2.
Send the new message immediately after sending the `VERACK` message.

Add support for receiving and parsing ADDRv2 messages.

Send ADDRv2 messages (instead of ADDR) to a peer if he has
advertised support for it.

Co-authored-by: Carl Dong <contact@carldong.me>

src/protocol.cpp

@@ -14,6 +14,8 @@ namespace NetMsgType {
 const char *VERSION="version";
 const char *VERACK="verack";
 const char *ADDR="addr";
+const char *ADDRV2="addrv2";
+const char *SENDADDRV2="sendaddrv2";
 const char *INV="inv";
 const char *GETDATA="getdata";
 const char *MERKLEBLOCK="merkleblock";
@@ -52,6 +54,8 @@ const static std::string allNetMessageTypes[] = {
     NetMsgType::VERSION,
     NetMsgType::VERACK,
     NetMsgType::ADDR,
+    NetMsgType::ADDRV2,
+    NetMsgType::SENDADDRV2,
     NetMsgType::INV,
     NetMsgType::GETDATA,
     NetMsgType::MERKLEBLOCK,