strip out unused and unneeded code

[pphaneuf]

A number of APIs (both external and internal) add burden to storage implementers, without bringing much benefits.

For example, QueueLeaves is complex to implement correctly, due to being a batching API with partial failure, but in practice, there are possibly no need for it, and due to the implementation complexity, is very unlikely to provide any performance benefit over using QueueLeaf (the non-batch version of this API) multiple times (potentially concurrently), say, and this both leaves the user in better control over what happens in the partial failure cases, and removes complexity of implementation (and the inevitable bugs that it brings).