limit the max size of entries sent per message.
Lower the cost at probing state as we limit the size per message;
lower the penalty when aggressively decrease to a too low next.
MultiNode is an alternative to raft.Node that is more efficient
when a node may participate in many consensus groups. It is currently
used in the CockroachDB project; this commit merges the
github.com/cockroachdb/etcd fork back into the mainline.