* Add support for Selector in tracker.Reference.
`tracker.Reference` may now optionally replace `Name` with `Selector` to track zero or multiple resources of a particular `apiVersion`, `kind` and `namespace`.
Fixes: https://github.com/knative/pkg/issues/859
* Address code review comments
`TrackReference` is the same as `Track`, but takes a `tracker.Reference` instead. This type has been seeded with the subset of `corev1.ObjectReference` that the tracker currently consumes / supports, but the intention is to expand this type to allow inexact references that (optionally) use label selectors in place of name to reference objects.
See also: https://github.com/knative/pkg/issues/859