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.
it is unfortunately. it has to be there because `config.Root` is not populated in builtins.