Implement exponential backoff for 500-599 status codes

[gen1us2k]

Errors in the 500-599 range (such as 500 or 503) indicate that there was an internal error in the FCM connection server while trying to process the request, or that the server is temporarily unavailable (for example, because of timeouts). Sender must retry later, honoring any Retry-After header included in the response. Application servers must implement exponential back-off.