Introduce instance ready hook

[Luit]

For the readiness tracking of Lightning Stream Enterprise we need a hook that checks snapshots (of any (Snapshot).Kind) to ensure health checks work properly and the "only_once" option waits until applying everything instead of only SnapshotKind.

[Luit]

Note: the LoadOnce is now done before managing waitingForInstances because that seemed more logical to me. It wasn't necessary for the implementation because, as the comment on the new hook implies, the check should work independently of whether the update it's currently processing has been applied or not. If I hadn't reorganised this, the diff would only be 10-ish lines.

[nvaatstra]

LGTM; Looks like a sensible place to include this new hook