rafthttp logs repeated messages when amounts of message-drop logs
happen, and it becomes log spamming.
Use MergeLogger to merge log lines in this case.
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.
Add remotes to rafthttp, who help newly joined members catch up the
progress of the cluster. It supports basic message sending to remote, and
has no stream connection for simplicity. remotes will not be used
after the latest peers have been added into rafthttp.
