Integrate raft's new feature (async write) into etcd

[ahrtr]

What would you like to be added?

raft introduced a new feature async write, etcd should try to integrate the new feature into etcd.

Why is this needed?

Improve performance

[yashkundu]

Hi @ahrtr, @jmhbnz I really want to work on this feature and contribute, can you please assign this issue to me?

[stale]

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.

[serathius]

Draft that I implemented some time ago #18027

[github-actions]

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.

[github-actions]

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.