Refactoring the timeout retries into a more general 'retry policy' with
support for retrying on HTTP 429 (Too Many Requests) and other responses
with a `Retry-After` header
The delay between requests is controlled by a combination of a 'backoff'
(currently only a simple linear backoff), and honouring the
`Retry-After` value (longest delay wins)
This makes the 'rate limiter' an optional argument as well, removing the
use of `nil` when one isn't supplied
Signed-off-by: David Ackroyd <daveo.ackroyd@gmail.com>
