- 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
