MINOR: session: add stick counters to the struct session

The stick counters in the session will be used for everything not related
to contents, hence the connections / concurrent sessions / etc. They will
be usable by "tcp-request connection" rules even without a stream. For now
they're just allocated and initialized.
diff --git a/src/stream.c b/src/stream.c
index 6730ad4..e21a2a5 100644
--- a/src/stream.c
+++ b/src/stream.c
@@ -107,6 +107,7 @@
 	sess->origin = &cli_conn->obj_type;
 	sess->accept_date = date; /* user-visible date for logging */
 	sess->tv_accept   = now;  /* corrected date for internal use */
+	memset(sess->stkctr, 0, sizeof(sess->stkctr));
 
 	if (unlikely((s = pool_alloc2(pool2_stream)) == NULL))
 		goto out_free_sess;