Create sized volume floc 978

Fixes FLOC-978
`service.create(name, size, ...)` does seem more natural to me, `service.get` is more an issue of arguably poor method naming, in that it's actually a method that returns a new `Volume` instance (which may or may not have already been created) that is owned by the calling `service`. I did this work based on @exarkun's design nodes and tbh, I'm not fully sure why we're eliminating creating a `Volume` instance in create; I'd have thought you could pass `create` a `name` and other optional parameters such as `size` and it could internally call `get` to get the `Volume` instance corresponding to those properties. 

Presumably there's some planned usage whereby we want to pass around a single `Volume` instance and have it `create`d in the storage pool and continue to work with that instance without getting a brand new `Volume` back, but I don't know what the design intention behind that is.