A small fix for extracting the md5 from a chunk file name on S3.

The existing method is preserved. If it fails, then it will fallback
to trying to find the last md5 before the '.sc' extension in the
file name.
Add 'is_private' config option for `to_s3_chunks`.

When 'is_private' is set, S3 files are verified using API calls (with
the provided credentials) rather than using an HTTP request.

Also includes some light refactoring: there's only one place in the code
where the md5 comparison is done for verification purposes.