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/sendf.c

@@ -419,8 +419,6 @@ CURLcode Curl_read(struct Curl_easy *data,   /* transfer */
   *n += nread;
   result = CURLE_OK;
 out:
-  /* DEBUGF(infof(data, "Curl_read(handle=%p) -> %d, nread=%ld",
-        data, result, nread)); */
   return result;
 }