Queue.get() no longer does get_children() and sorted() on every call

If the queue contains a large number of elements (> 10000), each `Queue.get()` will take a lot of time (>0.1sec.) due to frequent `get_children()` and `sorted()`.
Recips say that the call of `get_children() every time is not necessary:
https://zookeeper.apache.org/doc/r3.1.2/recipes.html#sc_recipes_Queues
```
 The client does not need to issue another getChildren( ) until it exhausts the list obtained from the first getChildren( ) call.
```
This patch implements this feature.