Merge bitcoin#19360: net: improve encapsulation of CNetAddr

bc74a40 net: improve encapsulation of CNetAddr (Vasil Dimov)

Pull request description:

  Do not access `CNetAddr::ip` directly from `CService` methods.

  This improvement will help later when we change the type of
  `CNetAddr::ip` (in the BIP155 implementation).

  (chopped off from bitcoin#19031 to ease review)

ACKs for top commit:
  dongcarl:
    ACK bc74a40
  naumenkogs:
    ACK bc74a40
  fjahr:
    Code review ACK bc74a40
  laanwj:
    code review ACK bc74a40
  jonatack:
    ACK bc74a40
  jnewbery:
    ACK bc74a40

Tree-SHA512: 29a203905538e8311e3249b78565abe69ce36dc4ec239bec85c726c30e1a7b55b0aaf5c6659b676935008e068cfa53d716f7a598469064108daf130f94329a5d

src/netaddress.cpp

@@ -726,12 +726,10 @@ bool CService::GetSockAddr(struct sockaddr* paddr, socklen_t *addrlen) const
  */
 std::vector<unsigned char> CService::GetKey() const
 {
-     std::vector<unsigned char> vKey;
-     vKey.resize(18);
-     memcpy(vKey.data(), ip, 16);
-     vKey[16] = port / 0x100; // most significant byte of our port
-     vKey[17] = port & 0x0FF; // least significant byte of our port
-     return vKey;
+    auto key = GetAddrBytes();
+    key.push_back(port / 0x100); // most significant byte of our port
+    key.push_back(port & 0x0FF); // least significant byte of our port
+    return key;
 }
 
 std::string CService::ToStringPort() const

src/netaddress.h

@@ -160,7 +160,11 @@ class CService : public CNetAddr
         CService(const struct in6_addr& ipv6Addr, uint16_t port);
         explicit CService(const struct sockaddr_in6& addr);
 
-        SERIALIZE_METHODS(CService, obj) { READWRITE(obj.ip, Using<BigEndianFormatter<2>>(obj.port)); }
+        SERIALIZE_METHODS(CService, obj)
+        {
+            READWRITEAS(CNetAddr, obj);
+            READWRITE(Using<BigEndianFormatter<2>>(obj.port));
+        }
 };
 
 bool SanityCheckASMap(const std::vector<bool>& asmap);