zarr v2 metadata access pattern for getitem

[ilan-gold]

At the moment there is something like

zarr-python/src/zarr/core/group.py

Lines 688 to 692 in 40da497

	zgroup_bytes, zarray_bytes, zattrs_bytes = await asyncio.gather(
	(store_path / ZGROUP_JSON).get(),
	(store_path / ZARRAY_JSON).get(),
	(store_path / ZATTRS_JSON).get(),
	)

to check for zarr v2 nodes. Would it be better to do attrs first and then the others? Do we think there is minimal overhead to doing all at once? I ask because previously, doing something like g["foo"] would only access .zarray if foo was a group and I'm looking to do the least fetching of data possible. Or maybe groups should be given precedence? I'm not sure how this used to work - I don't think I ever checked whether fetching group accessed .zarray previously.