metarepos: reduce commit delay

[hungryjang]

Since metarepos shares all information through raft, the commit delay is long. In particular, although there are many LS, the raft delay can become longer as report sharing increases.

To reduce commit delay, we propose a method of sharing only commit results between metarepos nodes. Reports are not shared through raft, but the collected reports are delivered to the leader through RPC. The leader builds a CommitResult based on the collected report and shares it through raft.