MINOR: channel: rename function chn_sess to chn_strm

The name of the function chn_sess is no longer appropriate.
This patch renames it to chn_strm.
diff --git a/src/stream.c b/src/stream.c
index 5583d1d..deb51e8 100644
--- a/src/stream.c
+++ b/src/stream.c
@@ -365,7 +365,7 @@
 	if (!(chn->flags & CF_ISRESP))
 		margin = global.tune.reserved_bufs;
 
-	s = chn_sess(chn);
+	s = chn_strm(chn);
 
 	b = b_alloc_margin(&chn->buf, margin);
 	if (b)