RoundTrip will retry a request if it receives REFUSED_STREAM. To guard
against servers that use REFUSED_STREAM to encourage rate limiting, or
servers that return REFUSED_STREAM deterministically for some requests,
we retry after an exponential backoff and we cap the number of retries.

The exponential backoff starts on the second retry, with a backoff
sequence of 1s, 2s, 4s, etc, with 10% random jitter.

The retry cap was set to 6, somewhat arbitrarily. Rationale: this is
what Firefox does.

Updates golang/go#20985

Change-Id: I4dcac4392ac4a3220d6d839f28bf943fe6b3fea7
Reviewed-on: https://go-review.googlesource.com/50471
Run-TryBot: Tom Bergan <tombergan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>