Deprecate v2 store

[gyuho]

Deprecate v2 (HTTP/1.1, in-memory database backend) in favor of v3 (HTTP/2, gRPC, memory-mapped on-disk storage backend).

Once we feel confident of our v2 emulation robustness #8473, deprecate v2 store and provide migration guide docs.

It's already announced as below:

https://coreos.com/blog/toward-etcd-v3-in-container-linux.html

With adoption coalescing around etcd v3, today we are announcing deprecation plans for older versions of etcd in CoreOS Container Linux (formerly known as CoreOS Linux). The current schedule for end of support for each version in Container Linux is:

etcd v0: May 7, 2017
etcd v2: February 1, 2018

etcd v2 scheduled for removal from Container Linux in February, 2018
Etcd v2 will be removed from the CoreOS Container Linux Alpha channel on February 1, 2018, and will not be supported after that time. As that version of Container Linux is promoted through the Beta and Stable channels, etcd v2 will be removed from them as well.

[stale]

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