etcdserver: move rpc defrag notifier into backend.

[siyuanfoundation]

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

the problem with the current implementation is:

1. there could potentially be >1 rpc servers (1 for secure, 1 for insecure).
2. each rpc server only sets its serve state based on defrag requests to its own server, unaware of each other. So one rpc server could still be serving traffic while the other requested defrag.

Based on these reasons, I think it is necessary to move the defrag notifiers further back to the backend.

[siyuanfoundation]

cc @chaochn47 @serathius

[ahrtr]

close/reopen to re-trigger all arm64 workflows

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed after 21 days if no further activity occurs. Thank you for your contributions.

[tjungblu]

/remove-lifecycle stale

[k8s-ci-robot]

@siyuanfoundation: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-etcd-unit-test-amd64	50c9868ab4132fabc2c73218edc686a961036769	link	true	/test pull-etcd-unit-test-amd64
pull-etcd-unit-test-arm64	50c9868ab4132fabc2c73218edc686a961036769	link	true	/test pull-etcd-unit-test-arm64

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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