Handling Downstream Deltas

[ricejasonf]

Since a Store may contain local changes, it would need to queue downstream deltas and squash them into one once it received confirmations from root of all the actions before it. nbdl::apply_action just returns a bool so this will not be enough to propagate the squashed delta.

A new function nbdl::apply_action_downstream_delta that returns an optional delta. It should be the basis for a new concept called LocalStore which implies that the store applies state changes before they are confirmed by the root server.

[ricejasonf]

not an issue