REORG: connection: rename app_cb "data"

Now conn->data will designate the data layer which is the client for
the transport layer. In practice it's the stream interface and will
soon also be the health checks.
diff --git a/src/stream_interface.c b/src/stream_interface.c
index 625ca73..faeddde 100644
--- a/src/stream_interface.c
+++ b/src/stream_interface.c
@@ -66,7 +66,7 @@
 	.chk_snd = stream_int_chk_snd_conn,
 };
 
-struct app_cb si_conn_cb = {
+struct data_cb si_conn_cb = {
 	.recv    = si_conn_recv_cb,
 	.send    = si_conn_send_cb,
 };