Support Retry-After header for rate-limited requests

The following discussion from !594 (merged) should be addressed:

• @steveazz started a discussion: (+2 comments)

  thought: Should we send when is the next time a request can be accepted? We usually send a set of documeneted headers which clients will be able to use programmatically for backoff. I think the most important one is Retry-After which is the standard header