Add Bulk Key/KeyValue iteration/retrieval

[ecton]

Currently there is no way to list all of the keys in the Key-Value store. The internal backup functionality uses internal APIs to perform this operation.

At a minimum we should have:

• A way to retrieve all keys
• A way to retrieve all keys and values
• A way to retrieve ranges of keys
• A way to retrieve ranges of keys and values

We should consider how retrieval should work with namespaced keys -- maybe it's an option whether to include child namespaces or not.