Upgrade etcd to v3.3.8, improvement: https://github.com/etcd-io/etcd/pull/9840

[humingcheng]

Fix high CPU usage when disconnected from etcd server

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[coveralls]

Coverage decreased (-0.1%) to 59.558% when pulling 831a20c on humingcheng:master into ab4ec2e on apache:master.