Integrate ActiveJob / DeliverLater / GlobalID with Rails

WIP.
Don't we already do that by providing `enqueue_at`, which isn't supported universally? The same argument could be made in Active Record for disallowing anything specific to a single database. While it's awesome to provide a common interface, and abstract away what we can, it doesn't make sense to me that we would hamstring basic functionality that is implemented in a similar enough fashion by almost all supported backends.