Use splice instead of erase/push_front

splice inserts an item into one container removing it from another
container. It seems to be what people use for this usecase in STL
LRU implementations.

Change-Id: I038aea2e3aadfce01ca30946c16fea3ce0b17b4f

zuul-preview/main.cc

@@ -69,8 +69,7 @@ public:
       return {};
 
     auto val = *(location->second);
-    queue.erase(location->second);
-    queue.push_front(val);
+    queue.splice(queue.begin(), queue, location->second);
     return val.second;
   }