Respect GitHub's Retry-After Header and other best-practices

[shahidhk]

ref: https://developer.github.com/guides/best-practices-for-integrators/#dealing-with-abuse-rate-limits

Quoting:

• Make authenticated requests, or use your application's client ID and secret. Unauthenticated requests are subject to more aggressive abuse rate limiting.
• Make requests for a single user or client ID serially. Do not make requests for a single user or client ID concurrently.
• If you're making a large number of POST, PATCH, PUT, or DELETE requests for a single user or client ID, wait at least one second between each request.
• When you have been limited, use the Retry-After response header to slow down. The value of the Retry-After header will always be an integer, representing the number of seconds you should wait before making requests again. For example, Retry-After: 30 means you should wait 30 seconds before sending more requests.
• Requests that create content which triggers notifications, such as issues, comments and pull requests, may be further limited and will not include a Retry-After header in the response. Please create this content at a reasonable pace to avoid further limiting.