The issue is not caused by this code, but by reading snapshot
from disk. etcd assumes the snapshot of v3 kv should live in
memory. If not, etcd does not work well anyway.
Use snapshotSender to send v3 snapshot message. It puts raft snapshot
message and v3 snapshot into request body, then sends it to the target peer.
When it receives http.StatusNoContent, it knows the message has been
received and processed successfully.
As receiver, snapHandler saves v3 snapshot and then processes the raft snapshot
message, then respond with http.StatusNoContent.
