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.
Also this is `go vet` warning, I suggest you to try `go vet` and `golint` for new code. You will probably  know some things about Go code style.