fix: retry logic

Adjust the retry logic so that it:

- Retries on 5XX server errors, 429 Too Many Requests, and 408 Request
  Timeout
- In the case of a 429, also parse the `Retry-After` header (if present)
  for the delay
- Tweak the exponential retry delay from millisecond decades starting
  from 1ms, to powers of 2 starting from 0.5s. It is unlikely that a 1ms
  delay will resolve the issue, but also going from 1s to 10s to 100s
  is quite a steep increase.

Also took the opportunity to upgrade reqwest to 0.13.

Signed-off-by: JP-Ellis <josh@jpellis.me>

Author: JP-Ellis