[Xline] refactor Range API

[themanforfree]

Our Range request is slower than etcd now, etcd's range request doesn't go through the consensus protocol, just ask the leader for a committed index, and then wait for the applied index of the current node to catch up with the committed index. and they can be batched when asking for index from leader. but in our current implementation, all Range requests use the curp protocol.
Xline
etcd