Back-port #259 to v0.9.8

Fix the caches mutating a deque node through a `NonNull` pointer derived from a
shared reference.

src/common/deque.rs

@@ -116,6 +116,7 @@ impl<T> Deque<T> {
         self.region
     }
 
+    #[cfg(any(test, feature = "sync", feature = "future"))]
     pub(crate) fn len(&self) -> usize {
         self.len
     }
@@ -272,6 +273,7 @@ impl<T> Deque<T> {
         std::mem::drop(Box::from_raw(node.as_ptr()));
     }
 
+    #[cfg(any(test, feature = "sync", feature = "future"))]
     pub(crate) fn reset_cursor(&mut self) {
         self.cursor = None;
     }

src/dash/base_cache.rs

@@ -1120,10 +1120,7 @@ where
             // invalidated ValueEntry (which should be still in the write op
             // queue) has a pointer to this node, move the node to the back of
             // the deque instead of popping (dropping) it.
-            if let Some(node) = deq.peek_front() {
-                let node = NonNull::from(node);
-                unsafe { deq.move_to_back(node) };
-            }
+            deq.move_front_to_back();
             true
         }
     }
@@ -1173,10 +1170,7 @@ where
                 // invalidated ValueEntry (which should be still in the write op
                 // queue) has a pointer to this node, move the node to the back of
                 // the deque instead of popping (dropping) it.
-                if let Some(node) = deqs.write_order.peek_front() {
-                    let node = NonNull::from(node);
-                    unsafe { deqs.write_order.move_to_back(node) };
-                }
+                deqs.write_order.move_front_to_back();
             }
         }
     }