Combine all etcd API guarantees into one

KV API guarantees
https://etcd.io/docs/v3.6/learning/api_guarantees/

Watch stream guarantee
https://etcd.io/docs/v3.6/learning/api/#watch-streams

Consider adding new "Bookmarkable" guarantee (or something like this) to provide guarantee about progress notification. We should document it in some way as it's required by K8s and should also never be broken.