net: improve encapsulation of CNetAddr.

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

 Adaptation of btc@bc74a40a56128f81f11151d5966f53b82f19038c

src/netaddress.cpp

@@ -666,12 +666,10 @@ 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;
-     vKey[17] = port & 0x0FF;
-     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

@@ -133,6 +133,7 @@ class CNetAddr
     uint32_t GetMappedAS(const std::vector<bool> &asmap) const;
 
     std::vector<unsigned char> GetGroup(const std::vector<bool> &asmap) const;
+    std::vector<unsigned char> GetAddrBytes() const { return {std::begin(ip), std::end(ip)}; }
     int GetReachabilityFrom(const CNetAddr* paddrPartner = nullptr) const;
 
     CNetAddr(const struct in6_addr& pipv6Addr, const uint32_t scope = 0);