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/store.rs

@@ -173,7 +173,10 @@ impl<B, N> Queue<B, N>
     ///
     /// If the stream is already contained by the list, return `false`.
     pub fn push(&mut self, stream: &mut store::Ptr<B>) -> bool {
+        trace!("Queue::push");
+
         if N::is_queued(stream) {
+            trace!(" -> already queued");
             return false;
         }
 
@@ -185,6 +188,8 @@ impl<B, N> Queue<B, N>
         // Queue the stream
         match self.indices {
             Some(ref mut idxs) => {
+                trace!(" -> existing entries");
+
                 // Update the current tail node to point to `stream`
                 let key = stream.key();
                 N::set_next(&mut stream.resolve(idxs.tail), Some(key));
@@ -193,6 +198,7 @@ impl<B, N> Queue<B, N>
                 idxs.tail = stream.key();
             }
             None => {
+                trace!(" -> first entry");
                 self.indices = Some(store::Indices {
                     head: stream.key(),
                     tail: stream.key(),