MINOR: stconn: Add counters to SC to know number of bytes received and sent

<bytes_in> and <bytes_out> counters were added to SC to count, respectively,
the number of bytes received from an endpoint or sent to an endpoint. These
counters are updated for connections and applets.

This patch is related to issue #1617.

include/haproxy/stconn-t.h

@@ -377,6 +377,9 @@ struct stconn {
 					      *    -1   : the SC is waiting for room but not on a specific amount of data
 					      *    >= 0 : min free space required to progress. 0 means SC must be unblocked ASAP
 					      */
+	unsigned long long bytes_in;         /* total number of bytes received from the SE */
+	unsigned long long bytes_out;        /* total number of bytes sent to the SE */
+
 	struct wait_event wait_event;        /* We're in a wait list */
 	struct sedesc *sedesc;               /* points to the stream endpoint descriptor */
 	enum obj_type *app;                  /* points to the applicative point (stream or check) */