Encapsulate retry logic

[jmgasper]

The handleEventGracefully logic needs to be moved out of the IssueService and into it's own class. For instance, we want the same retry logic to apply to creating the Topcoder challenge for a copilot payment, but that would currently require redundant code, so we need to get the retry logic better encapsulated to make it more useful to all the individual services, both current and future.