REORG: buffer: move buffer_flush, b_adv and b_rew to buffer.h

These one now operate over real buffers, not channels anymore.
diff --git a/src/stream_interface.c b/src/stream_interface.c
index 2066b06..14e2cd5 100644
--- a/src/stream_interface.c
+++ b/src/stream_interface.c
@@ -1071,7 +1071,7 @@
 					fwd = b->to_forward;
 				b->to_forward -= fwd;
 			}
-			b_adv(b, fwd);
+			b_adv(&b->buf, fwd);
 		}
 
 		if (conn->flags & CO_FL_WAIT_L4_CONN)