Picked - Support asynchronous storage writes

[tisonkun]

File an issue to track the upstream feature "async storage write" - etcd-io/raft#8

Open to comments on whether we should catch it up and if there is a plan.

[gengliqi]

raft-rs implemented async storage write in #403 three years ago.

[tisonkun]

@gengliqi Thanks for your information!