Callback module for implementing key expiration via Process.send_after. This approach works by using the BEAM’s internal scheduling expire keys. Process.send_after will wait for some period of time and then send a message to the cache server. The cache server is configured to handle :expire messages and will delegate back to this module which will ensure that we’re properly filtering out irrelevant expire messages (i.e. when we “refresh” the expiration we are just incrementing a pointer/watermark and we ignore any previous messages)