Honor rate limiting headers sent by the server

[ePaul]

Current behavior

Nakadi (at least on the staging system at Zalando) now has some rate limiting (via a gateway).
This has the effect that all kinds of calls can get 429 responses with a Retry-After header.

When they happen during a cursor commit, Nakadi-Java just throws them out of the StreamOffsetObserver.onNext() method (see #357).
If those exceptions are caught (as our application is doing right now, apparently), this is just continued with the next messages, resulting in ~25k log lines over 45 seconds, until the rate limit window is over.

Expected behavior
For a 429 response, the Retry-After header is evaluated. After that many seconds, the request is tried again.

[ePaul]

Also, it would be helpful to actually print the Retry-After header when logging these exceptions.