Set Message::default_length() to 128.

This is experimental change to reduce static memory footprint of very
large dataflow graphs.  Allocating 1024 entries per channel adds up to
tens of MB even for a single worker.  The new setting will likely have
adverse effect on performance in some scenarios.  Ideally, we would like
an adaptive channel size that scales up an down with demand.

timely/src/dataflow/channels/mod.rs

@@ -28,7 +28,7 @@ pub struct Message<T, D> {
 impl<T, D> Message<T, D> {
     /// Default buffer size.
     pub fn default_length() -> usize {
-        1024
+        128
     }
 
     /// Creates a new message instance from arguments.