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.
I wonder why we can't just use `daemon.Links` as field