Inserted break when iterating Route53 records.

``find_records()`` would iterate through the entire zone even though there
were no more matching records since records are returned sorted. Added
break statement so iteration stops after there is no more matching
records to return.
Added backoff support for route53 throttling.

Instead of erroring out on 400-Throttling responses from route53,
perform exponential backoff retries on the request.
Add test for iteraton break of Route53 records
Merge pull request #2631 from kyleknap/route53

Inserted break when iterating Route53 records.