zulip: Optionally retry requests that hit RATE_LIMIT_HIT.

[devnishantt]

HTTP 429 currently bubbles up to every application. Tim asked for this
as an opt-in Client option so callers that cannot block keep the old
behavior.

When auto_retry_rate_limits=True, do_api_query sleeps for the
server's retry-after (JSON field, then Retry-After header) plus a small
jitter, then retries, up to 10 times. The cap and jitter avoid pinning
a client to the server's allowed rate.

Default is False.

Fixes #773.

Previous #908 implemented a simpler version of this and was closed
unreviewed.

How did you test this PR?
Unit tests in zulip/tests/test_rate_limit.py (7 passed): default off,
JSON retry-after, Retry-After header fallback, jitter, 10-retry cap.

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