Return an instance of S3Store.

def get_s3store_instance(bucket):
    """Return an instance of S3Store."""
    global _s3store_instances

    key = "%s" % bucket

    try:
        instance = _s3store_instances[key]
    except KeyError:
        instance = S3Store(
            bucket=bucket
        )
        _s3store_instances[key] = instance

    return instance