Elide endpoint probes when the set of services does not change

[mattmoor]

A good example of this would be updating traffic: to change percentages (or @vagababov new gradual rollouts) where the set of services in generation: X and generation: X+1 are identical.

We get the former from oldGeneration and the latter from ing here: https://github.com/knative-sandbox/net-contour/blob/7bff03576e1c01114d47c0fdc58dac74a98115db/pkg/reconciler/contour/contour.go#L119

The simplest correct solution is that when the set of services is identical per visibility to elide the endpoint probe, but we could perhaps optimize more generally by excluding services present in both sets and simply eliding when that leaves the empty set 🤔

[github-actions]

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[dprotaso]

/lifecycle frozen