Time-out DynamoDB chunk requests faster

[bboreham]

I see occasional DynamoDB.BatchGetItemPage requests that take tens of seconds, where the typical time is under 100ms:

suggest putting a timeout like 2 seconds on these so they will go round the retry loop and hopefully have better luck.

I think this can be done via ResponseHeaderTimeout on the Transport

[alvinlin123]

As chunk store is deprecated, I think we should close this issue. Please feel free to re-open if needed.