Retries

[vjkoskela]

In the events of a non-200 (except possibly 400) or an IOException from the Apache Http Client we should enqueue the event for retry. We can either use a second queue which wraps the event with a retry counter (starting from a configurable number of retries) and a backoff (starting from a configurable interval). Alternatively, we can extend the primary queue into a priority queue to account for this. I suspect the former is more straight forward. The priority queue can also be implemented on a per thread basis thus ensuring it does not need to be thread safe.