store: add a mechanism to summarize changes

[nicks]

Hello @maiamcc, @landism, @milas,

Please review the following commits I made in branch nicks/summarize:

322e263 (2021-03-04 18:20:51 -0500)
store: add a mechanism to summarize changes
The goal of this is to help us move more towards a system like
Kubernetes (https://pkg.go.dev/sigs.k8s.io/controller-runtime@v0.8.2/pkg/reconcile#Request)
where the system tells us which object is changing, rather
than writing lots of code to figure it out on our own.

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[milas]

This looks fine to me, but I do wonder if given the goal is

the system tells us which object is changing

Would it be easier to have something like a TypedAction interface that has Types() []schema.GroupVersionKind (could also restrict to 1x type) and then just pass the list of kinds to OnChange (which would be nil for any "legacy" events that don't implement the iface)? (Alternatively, could also have TypedSubscriber interface which also provides a list of kinds it cares about so that the notifier could handle the filtering.)

[nicks]

Ya, that might be where this object ends up - with a list of types. But I'm pretty sure it's going to need to be more complex than that. For example,

• actions can be batched together, and there are cases where you'll need to know if there are ANY legacy actions
• the Reconcile() function tends to expect the name of the object that changed