Remove cancelled pushes and shifts at the end of every processOrders() call;

lib/index.js

@@ -104,6 +104,20 @@ const Channel = function(length = 0) {
 
     pushes.splice(0, index.push);
     shifts.splice(0, index.shift);
+
+    // Remove cancelled pushes
+    for (let i = pushes.length - 1; i >= 0; i--) {
+      if (pushes[i].cancelled) {
+        pushes.splice(i, 1);
+      }
+    }
+
+    // Remove cancelled shifts
+    for (let i = shifts.length - 1; i >= 0; i--) {
+      if (shifts[i].cancelled) {
+        shifts.splice(i, 1);
+      }
+    }
   };
 
   const readOnly = Object.freeze(