Understand rate-limiting responses (429, RATE_LIMIT_HIT)

[gnprice]

The Zulip server rate-limits requests. By default the limit for each user is 200 per minute, in total across all types of requests. For an example where this came up, see #945 and its original report.

We'll resolve #945 by having fetchOlder specifically add backoff for the getMessages requests we make there. But then a further bonus refinement would be for us to understand when the server is specifically giving us rate-limiting errors, and follow its instructions for how long to wait before making another request.

For docs on the rate-limiting errors:

• Partial documentation is here: https://zulip.com/api/rest-error-handling#rate-limit-exceeded
• Some other aspects appear here: https://zulip.com/api/http-headers#rate-limiting-response-headers
• Though we don't seem to have this part documented, the HTTP status code is 429, which is standard: https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/429

To do this right, this logic belongs on ApiConnection. That way the information can be shared across all the different requests we make on behalf of a given account.