[mplex] Add benchmark, tweak default split_send_size. (#1834)

* [mplex]  Benchmark different split_send_size configurations.

With both TCP and memory transports. As a result, change the
default `split_send_size` to 8KiB.

* Cleanup

* [mplex] Update CHANGELOG.

* Add throughput information.

muxers/mplex/src/config.rs

@@ -119,7 +119,7 @@ impl Default for MplexConfig {
             max_substreams: 128,
             max_buffer_len: 32,
             max_buffer_behaviour: MaxBufferBehaviour::Block,
-            split_send_size: 1024,
+            split_send_size: 8 * 1024,
         }
     }
 }