Unify four backoff implementations

[bboreham]

Fixes #609

As an added bonus the wait in aws_storage_client ends early if the Context is cancelled.

[jml]

Please write godoc for all of these public types & methods.

[tomwilkie]

Only minor comment: everywhere this is it used, the meaning is inverted (we're asking NotFinished). Perhaps inverting this function would make a slightly nicer interface? Call it Remaining or something?

[tomwilkie]

LGTM modulo one nit.

[bboreham]

I went with Ongoing() instead of !Finished(), and found another couple of backoff implementations to replace. PTAL.

[tomwilkie]

I wonder is Ongoing should check if done is closed, and then replace the call to isClosed here with a call to Ongoing?

[bboreham]

From the Context docs we would check Err() != nil

That would change the semantics; probably in an ok way, but I felt this was conflating the two mechanisms too much and we should check explicitly in the outer loop.

I have another PR under way where I'm doing that. Maybe I'll change my mind.

[bboreham]

OK, that comment didn't make very much sense in this context. See #618 for what I meant.

[tomwilkie]

:-) Thanks Brian. I'll try and look at that tomorrow, but I'm flying then, so may be a little delayed.

[tomwilkie]

Nice. On question, definitely not very important. Feel free to address or not, no need for another review.