v2.0.0: Fix delayed timestamp set

This is a breaking change

When comparing this project to ruby resque, we had been using the wrong value (key pointer) in the delayed job queue. When running this project alongside a ruby scheduler, it was possible for older timestamp key set entries not to be deleted. This update corrects the difference.

When upgrading to this new version, it is very likely you will end up with queue artifacts that are not properly deleted, which have the potential to break plugin's behaviors. This will require manual intervention to remove keys that match resque:timestamps:*. Before upgrading to this version, the safest path forward is to drain all of your queues and schedules first, using the previous version of node-resque.

by @evantahler via #121