change default storage_type to fs (from s3)
demonstrate propagating result to s3
BUGFIX: give reduce default

so passing empty list as argument works
refactor for clarity and speed

create separate _s3_{read,write} functions that interact only with s3 objects
enable passing bucket rather than bucket str to s3_{read,write}_result: reduce s3 connection creations
use new s3 helper to write result list