MINOR: connection: introduce conn_stream

This patch introduces a new struct conn_stream. It's the stream-side of
a multiplexed connection. A pool is created and destroyed on exit. For
now the conn_streams are not used at all.
diff --git a/src/haproxy.c b/src/haproxy.c
index ed6c8ba..d3db264 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -2189,6 +2189,7 @@
 	pool_destroy2(pool2_stream);
 	pool_destroy2(pool2_session);
 	pool_destroy2(pool2_connection);
+	pool_destroy2(pool2_connstream);
 	pool_destroy2(pool2_requri);
 	pool_destroy2(pool2_task);
 	pool_destroy2(pool2_capture);