Make queue item list caching period configurable.

With 400+ executors, I am seeing a lot of HTTP threads blocked at:
```
    java.lang.Thread.State: BLOCKED (on object monitor)
        at hudson.model.Queue.getItems(Queue.java:691)
        - waiting to lock <0x000000040768ba18> (a hudson.model.Queue)
        at hudson.model.Queue$CachedItemList.get(Queue.java:218)
        at hudson.model.Queue.getApproximateItemsQuickly(Queue.java:721)
```