Merge #20146: net: Send post-verack handshake messages at most once

fa1f6f2 net: Send post-verack handshake messages at most once (MarcoFalke)

Pull request description:

  There is no need to send `SENDHEADERS` and `SENDCMPCT` messages as a reply to each `VERACK` that is received. For alive checks, a `PING`/`PONG` can be used.

ACKs for top commit:
  jonatack:
    Concept ACK fa1f6f2 this is the only code section that sets `fCurrentlyConnected` and `fSuccessfullyConnected` to true. Could add a test. I did not verify if this code is actually being called repeatedly post initial verack; was it?
  hebasto:
    ACK fa1f6f2, I have reviewed the code and it looks OK, I agree it can be merged.
  naumenkogs:
    ACK fa1f6f2
  laanwj:
    Code review ACK fa1f6f2

Tree-SHA512: c841d5d3807254a49463bbcfac3b32881b34a9d3206899544c86322c20988e17ad2ae243cba227fd3825a914f0cb2584451edda2414aecee6d5e3f5a0636f08a

src/net_processing.cpp

@@ -2462,8 +2462,9 @@ void PeerManager::ProcessMessage(CNode& pfrom, const std::string& msg_type, CDat
     // At this point, the outgoing message serialization version can't change.
     const CNetMsgMaker msgMaker(pfrom.GetCommonVersion());
 
-    if (msg_type == NetMsgType::VERACK)
-    {
+    if (msg_type == NetMsgType::VERACK) {
+        if (pfrom.fSuccessfullyConnected) return;
+
         if (!pfrom.IsInboundConn()) {
             // Mark this node as currently connected, so we update its timestamp later.
             LOCK(cs_main);