Remove datastore.key._dataset_ids_equal and just use string equality check

[dhermes]

See #528 for some history here.

The issue is that requests made using foo instead of s~foo fail in strange ways.

So as a result we have to use datastore.helpers._prepare_key_for_request to strip dataset IDs from keys that get sent to the server. This has accidental caused bugs in the past (e.g. allowing keys from a different dataset bar to be inserted into dataset foo)

[lukesneeringer]

@dhermes Can you advise on this one?

[dhermes]

@tseaver @lukesneeringer This is no longer relevant (I think because of changes made in the latest version of the API)