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/ws.c

@@ -987,8 +987,7 @@ CURL_EXTERN CURLcode curl_ws_send(struct Curl_easy *data, const void *buffer,
     return CURLE_SEND_ERROR;
   }
   if(!data->conn->proto.ws) {
-    failf(data, "Not a websocket transfer on connection #%ld",
-          data->conn->connection_id);
+    failf(data, "Not a websocket transfer");
     return CURLE_SEND_ERROR;
   }
   ws = data->conn->proto.ws;