Update metadata document keys for V3 specification #1515

The experimental V3 implementation in Zarr-Python included metadata keys (e.g. /meta/foo/bar.group.json) that 1) were separate from chunks and 2) included either the array or group designator in the key name. This key schema was subsequently changed and needs to be updated in Zarr-Python.

In the final spec, both group and array keys are simply called zarr.json (e.g. /foo/bar/zarr.json). A key in the metadata document (node_type) is now used to differentiate between group and array metadata documents.