Add read_mbytes and write_mbytes from statistics

[paultag]

This is a bit awkward at the moment, and should perhaps be refactored
into more statistical helpers, but for now, being able to grab the
read/write count to the device is fairly handy.

[paultag]

I put this into StorageObject - I'm not sure if this depends on a particular backstore. If so, I'll pull this into either a function or a mixin that we can throw into the supported backstores.

Just a flag if anyone knows

[mikechristie]

Would it be possible to make it more generic similar to how we get attributes/params?

For example, check out node.py:list_attributes() for how to loop over the statistics dir and put all of them in an array. We could then also do something like node.py:get_attribute() to get individual stats if needed.

I think we could get this to work at the storage object and target levels similar to how the attribute code works.