RFE: Explore in-process v3 etcd server

[timothysc]

Recent changes to etcd3 have made it easier to embed the etcd server into the executable: https://github.com/coreos/etcd/pull/7496/files

Before that, there was little motivation other then deployment and distribution. This would allow for an easier deployment model and increased performance for in-process embedding(sans-transport), which may even lead to cache-reduction or elimination.

The tradeoff here is the apiserver will need to be deployed in 1,3,5.

As we talk about kubeadm-HA deployments we may want to consider this approach.

/cc @kubernetes/sig-scalability-feature-requests @kubernetes/sig-cluster-lifecycle-feature-requests

[xiang90]

/cc @justinsb

[wojtek-t]

Removing milestone - this isn't critical and noone really has capacity for it.

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