Links v2: Migrate Links to Jobs, with supporting API.

This migrates almost all links operations to jobs. There were a couple of places I haven't been able to resolve cleanly in `daemon/daemon.go`, but these will need an independent evaluation.

This passes all tests, is exercised (hopefully) fully, and should be suitable for merge after review. PTAL, thanks.
Maybe we should call such jobs just `job`? Because it's pretty obvious from context what the job it is and it will be easier for `copy-paste development`