Move all Interface methods to be package methods taking context

[riannucci]

So instead of:

ds := datastore.Get(c)
err := ds.Get(model)
err := ds.Get(model)

it would be

err := ds.Get(c, model)
err := ds.Get(c, model)

This would allow per-rpc timeouts to be a lot less awkward (since you're passing the context in).

This would be done to all services (adding RawInterface if necessary for services that don't split them apart).

[riannucci]

suggested by @vadimsht