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

include/curl/curl.h

@@ -2924,7 +2924,9 @@ typedef enum {
   CURLINFO_REFERER          = CURLINFO_STRING + 60,
   CURLINFO_CAINFO           = CURLINFO_STRING + 61,
   CURLINFO_CAPATH           = CURLINFO_STRING + 62,
-  CURLINFO_LASTONE          = 62
+  CURLINFO_XFER_ID          = CURLINFO_OFF_T + 63,
+  CURLINFO_CONN_ID          = CURLINFO_OFF_T + 64,
+  CURLINFO_LASTONE          = 64
 } CURLINFO;
 
 /* CURLINFO_RESPONSE_CODE is the new name for the option previously known as