net: prevent sending messages in NetEventsInterface::InitializeNode

Now that the queueing of the VERSION messages has been moved out of
`InitializeNode`, there is no need to pass a mutable `CNode` reference any
more. With a const reference, trying to send messages in this method would
lead to a compile-time error, e.g.:

----------------------------------------------------------------------------------------------------------------------------------
...
net_processing.cpp: In member function ‘virtual void {anonymous}::PeerManagerImpl::InitializeNode(const CNode&, ServiceFlags)’:
net_processing.cpp:1683:21: error: binding reference of type ‘CNode&’ to ‘const CNode’ discards qualifiers
 1683 |     PushNodeVersion(node, *peer);
...
----------------------------------------------------------------------------------------------------------------------------------

Github-Pull: bitcoin#30394
Rebased-From: 0dbcd4c

src/net.h

@@ -1000,7 +1000,7 @@ class NetEventsInterface
     static Mutex g_msgproc_mutex;
 
     /** Initialize a peer (setup state) */
-    virtual void InitializeNode(CNode& node, ServiceFlags our_services) = 0;
+    virtual void InitializeNode(const CNode& node, ServiceFlags our_services) = 0;
 
     /** Handle removal of a peer (clear state) */
     virtual void FinalizeNode(const CNode& node) = 0;

src/net_processing.cpp

@@ -502,7 +502,7 @@ class PeerManagerImpl final : public PeerManager
         EXCLUSIVE_LOCKS_REQUIRED(!m_most_recent_block_mutex);
 
     /** Implement NetEventsInterface */
-    void InitializeNode(CNode& node, ServiceFlags our_services) override EXCLUSIVE_LOCKS_REQUIRED(!m_peer_mutex);
+    void InitializeNode(const CNode& node, ServiceFlags our_services) override EXCLUSIVE_LOCKS_REQUIRED(!m_peer_mutex);
     void FinalizeNode(const CNode& node) override EXCLUSIVE_LOCKS_REQUIRED(!m_peer_mutex, !m_headers_presync_mutex);
     bool HasAllDesirableServiceFlags(ServiceFlags services) const override;
     bool ProcessMessages(CNode* pfrom, std::atomic<bool>& interrupt) override
@@ -1566,7 +1566,7 @@ void PeerManagerImpl::UpdateLastBlockAnnounceTime(NodeId node, int64_t time_in_s
     if (state) state->m_last_block_announcement = time_in_seconds;
 }
 
-void PeerManagerImpl::InitializeNode(CNode& node, ServiceFlags our_services)
+void PeerManagerImpl::InitializeNode(const CNode& node, ServiceFlags our_services)
 {
     NodeId nodeid = node.GetId();
     {