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

### What would you like to be added?

raft introduced a new feature [async write](https://github.com/etcd-io/raft/pull/8), etcd should try to integrate the new feature into etcd.

### Why is this needed?

Improve performance

Start by reading the linked raft async-write change and locating etcd's raft write path. Determine the integration points and any required validation for etcd; done means the async-write feature is integrated into etcd and improves performance as intended.

Written by the indexing model from the issue text.