lib: add CURLINFO_CONN_ID and CURLINFO_XFER_ID

- add an `id` long to Curl_easy, -1 on init
- once added to a multi (or its own multi), it gets
  a non-negative number assigned by the connection cache
- `id` is unique among all transfers using the same
  cache until reaching LONG_MAX where it will wrap
  around. So, not unique eternally.
- CURLINFO_CONN_ID returns the connection id attached to
  data or, if none present, data->state.lastconnect_id
- variables and type declared in tool for write out

Closes #11185

lib/easy.c

@@ -893,6 +893,8 @@ struct Curl_easy *curl_easy_duphandle(struct Curl_easy *data)
   /* the connection cache is setup on demand */
   outcurl->state.conn_cache = NULL;
   outcurl->state.lastconnect_id = -1;
+  outcurl->state.recent_conn_id = -1;
+  outcurl->id = -1;
 
   outcurl->progress.flags    = data->progress.flags;
   outcurl->progress.callback = data->progress.callback;