_iter_chunk_keys iterates over the wrong shape

[d-v-b]

>>> import zarr
>>> store = {}
>>> z = zarr.create_array(store=store, shape=(10,), chunks=(1,), shards=(3,))
>>> z[:] = 5
>>> tuple(z._iter_chunk_keys())
('c/0', 'c/1', 'c/2', 'c/3', 'c/4', 'c/5', 'c/6', 'c/7', 'c/8', 'c/9')
>>> store.keys()
dict_keys(['zarr.json', 'c/0', 'c/1', 'c/2', 'c/3'])

The solution is to fix _iter_chunk_coords to iterate over the shard shape, not the chunk shape.

xref #3217

[ziw-liu]

Is this related to #3221?

[d-v-b]

yes, and like #3221 it should be addressed by #3299