If we've hit the maximum number of retry attempts, return `true` for
`done`. As it stands, it is possible for us to return a `nil` error
(e.g. if the request sends successfully but returns a `50x` status) and
`false` for `done`, which `retry.Until` will interpret as "keep
retrying".
Not sending request body on retry is bad enough, but we keep the original
`Content-Length` and the server ends up expecting a body we never send.
This results in `400 Bad Request` errors from the ALB.
We've seen failures in CI where DNS lookups fail which cause our
operations against the service to fail, as well as other sorts of
timeouts.
Add a set of helper methods in a new httputil package that helps us do
retries on these operations, and then update our client library to use
them when we are doing GET requests. We also provide a way for non GET
requests to be retried, and use this when updating a lease (since it
is safe to retry multiple requests in this case).
