Send final alert message to older peers after connecting.

The old Bitcoin alert system has long since been retired.
( See also: https://bitcoin.org/en/alert/2016-11-01-alert-retirement )

This change causes each node to send any old peers that
 it connects with a copy of the final alert.

The alert it hardcode cancels all other alerts including
 other final alerts.

src/net_processing.cpp

@@ -1324,6 +1324,12 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
         pfrom->nTimeOffset = nTimeOffset;
         AddTimeData(pfrom->addr, nTimeOffset);
 
+        // If the peer is old enough to have the old alert system, send it the final alert.
+        if (pfrom->nVersion <= 70012) {
+            CDataStream finalAlert(ParseHex("60010000000000000000000000ffffff7f00000000ffffff7ffeffff7f01ffffff7f00000000ffffff7f00ffffff7f002f555247454e543a20416c657274206b657920636f6d70726f6d697365642c2075706772616465207265717569726564004630440220653febd6410f470f6bae11cad19c48413becb1ac2c17f908fd0fd53bdc3abd5202206d0e9c96fe88d4a0f01ed9dedae2b6f9e00da94cad0fecaae66ecf689bf71b50"), SER_NETWORK, PROTOCOL_VERSION);
+            connman.PushMessage(pfrom, CNetMsgMaker(nSendVersion).Make("alert", finalAlert));
+        }
+
         // Feeler connections exist only to verify if address is online.
         if (pfrom->fFeeler) {
             assert(pfrom->fInbound == false);