MINOR: channel: rename bi_erase() to channel_truncate()

It applies to the channel and it doesn't erase outgoing data, only
pending unread data, which is strictly equivalent to what recv()
does with MSG_TRUNC, so that new name is more accurate and intuitive.
diff --git a/src/stream_interface.c b/src/stream_interface.c
index f05f4a8..9025851 100644
--- a/src/stream_interface.c
+++ b/src/stream_interface.c
@@ -125,8 +125,8 @@
 	channel_abort(si->ib);
 	channel_auto_close(si->ib);
 	channel_erase(si->ib);
+	channel_truncate(si->ob);
 
-	bi_erase(si->ob);
 	if (likely(msg && msg->len))
 		bo_inject(si->ob, msg->str, msg->len);