If the request to conditionally update a lease counter in DynamoDB fails, it's
considered a failure to renew the lease. This is a good thing, except if the
request failure was just because of connectivity problems. In this case the
counter *did* update in DynamoDB, but the Dynamo client retries the request
which then fails the update condition (since the lease counter no longer
matches expected value).
To handle this gracefully we opt to get the lease record from Dynamo and
examine the lease owner and counter. If it matches what we were expecting,
then we consider renewal a success.
