Add the ability to pause/resume s3util uploads

[schmichael]

Problem:

Right now there's no way to gracefully shutdown a process mid-upload. The upload struct is private - as are a number of key pieces of state.

Proposed Solution:

Add a Pause method and a Resume function to allow for graceful shutdowns and starting back up.

Implementation details:

• The Public API has not changed. However, I've expanded the breadth of the public API which may make compatibility harder to maintain in the future.
• The return value of Pause, UploaderState, makes every field public to allow users to marshal it as they see fit. (Although I haven't written custom marshaling code yet, so I wonder if the part type will need to be made public to support some use cases.)
• Resume requires a Config in addition to the UploaderState since http clients can't be automatically marshalled (not to mention callers may not want their sensitive credentials included in the uploader state).
• I wrote a number of integration tests locally using my own S3 keys, but I didn't commit them because they're slow and require S3 keys. If you're curious, here's most of the code: https://gist.github.com/schmichael/a888d7ce5a0a9182d7c9

[schmichael]

Just cleaning up old cruft of mine. 👋