Auto-refresh S3 credentials for Store.get_s3fs_filesystem

[chuckwondo]

Currently, the method Store.get_s3fs_filesystem returns an s3fs.S3FileSystem instance with a default AWS session that does not automatically refresh its credentials when the credentials expire.

This should be rectified by constructing a session object and passing it into the s3fs.S3FileSystem constructor rather than explicitly passing key, secret, and token, where the session is configured to automatically refresh the credentials.