Backport of net 2898, graceful_startup into release/1.0.x

[hc-github-team-consul-core]

Backport

This PR is auto-generated from #239 to be assessed for backporting due to the inclusion of the label backport/1.0.

WARNING automatic cherry-pick of commits failed. Commits will require human attention.

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The below text is copied from the body of the original PR.

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Adding graceful_startup endpoint and function that will be called by dataplane postStart hook. Graceful_startup will block all other containers from starting up until envoy dataplane is determined to be ready.

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Overview of commits

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