net: Use actual memory size in receive buffer accounting

Add a method CNetMessage::GetMemoryUsage and use this for accounting of
the size of the process receive queue instead of the raw message size.

This ensures that allocation and deserialization overhead is taken into
account.

src/net.h

@@ -245,6 +245,9 @@ public:
     CNetMessage(const CNetMessage&) = delete;
     CNetMessage& operator=(CNetMessage&&) = default;
     CNetMessage& operator=(const CNetMessage&) = delete;
+
+    /** Compute total memory usage of this object (own memory + any dynamic memory). */
+    size_t GetMemoryUsage() const noexcept;
 };
 
 /** The Transport converts one connection's sent messages to wire bytes, and received bytes back. */