net: add I2P to the reachability map

Update `CNetAddr::GetReachabilityFrom()` to recognize the I2P network so
that we would prefer to advertise our I2P address to I2P peers.

src/netaddress.cpp

@@ -878,6 +878,11 @@ int CNetAddr::GetReachabilityFrom(const CNetAddr *paddrPartner) const
         case NET_IPV4:   return REACH_IPV4; // Tor users can connect to IPv4 as well
         case NET_ONION:    return REACH_PRIVATE;
         }
+    case NET_I2P:
+        switch (ourNet) {
+        case NET_I2P: return REACH_PRIVATE;
+        default: return REACH_DEFAULT;
+        }
     case NET_TEREDO:
         switch(ourNet) {
         default:          return REACH_DEFAULT;