Fix pipe tests

I don't really understand why the size is that high.
The size of the `Message` struct is 2416.
The channel is a message in an unsafecell + 1 atomicU8 and 2 atomic bools.
It should only 3 more bytes but for some reason it's 16

src/pipe.rs

@@ -45,7 +45,7 @@ mod tests {
     // size.  Bumping the size is not a breaking change but should only be done if really
     // necessary.
 
-    const MAX_SIZE: usize = 2416;
+    const MAX_SIZE: usize = 2432;
 
     fn assert_size<T>() {
         let size = mem::size_of::<T>();
@@ -65,6 +65,7 @@ mod tests {
     #[test]
     fn test_interchange_size() {
         use interchange::Channel;
+        // The real cost per-client
         assert_size::<Channel<Request, Reply>>();
     }
 }