urldata: move async resolver state from easy handle to connectdata

- resolving is done for a connection, not for every transfer
- save create/dup/free of a cares channel for each transfer
- check values of setopt calls against a local channel if no
  connection has been attached yet, when needed.

Closes #12198

lib/multi.c

@@ -1986,8 +1986,8 @@ static CURLMcode multi_runsingle(struct Curl_multi *multi,
 
       if(dns) {
 #ifdef CURLRES_ASYNCH
-        data->state.async.dns = dns;
-        data->state.async.done = TRUE;
+        conn->resolve_async.dns = dns;
+        conn->resolve_async.done = TRUE;
 #endif
         result = CURLE_OK;
         infof(data, "Hostname '%s' was found in DNS cache", hostname);