[Etcd downgrade] Implement downgrade validate, enable and cancel#11801

Merged

gyuho merged 2 commits intoetcd-io:masterfrom YoyinZyc:downgrade-server

May 18, 2020

+211-20

Commits on May 15, 2020

etcdserver: define error string and link with grpc error code.
YoyinZyc
committed
etcdserver: handle and apply downgrade validate, enable and cancel requests
YoyinZyc
committed