This PR sets etcd version and min cluster version in request header,
and let server check version compatibility. rafthttp server
will reject any message from peer with incompatible version(too low
version or too high version), and print out warning logs.
This helps etcd 2.1 connect to msgappV1 handler when the remote member
doesn't support msgappV2. And it doesn't print out unsupported handler
error to make log clean.
