Do not reuse next ptr for multiple linked lists

Because, you might think that each linked list has exclusive access to
the next pointer, but then there is an edge case that proves otherwise.
Also, debugging this kind of thing is annoying.

src/proto/streams/recv.rs

@@ -31,7 +31,7 @@ pub(super) struct Recv<B> {
     pending_window_updates: store::Queue<B, stream::NextWindowUpdate>,
 
     /// New streams to be accepted
-    pending_accept: store::Queue<B, stream::Next>,
+    pending_accept: store::Queue<B, stream::NextAccept>,
 
     /// Holds frames that are waiting to be read
     buffer: Buffer<Bytes>,