Honor Retry-After

[cstar]

From table 9 in FCM manual:

The server couldn't process the request in time. Retry the same request, but you must:

• Honor the Retry-After header if it is included in the response from the FCM Connection Server.
• Implement exponential back-off in your retry mechanism. (e.g. if you waited one second before the first retry, wait at least two second before the next one, then 4 seconds and so on). If you're sending multiple messages, delay each one independently by an additional random amount to avoid issuing a new request for all messages at the same time.

Either implement the two options directly in the worker or update GCM.NotificationResponseto help consumer make informed decisions.