raft: support to do multiple proposals in one message
rafthttp: batch MsgProp

If amounts of MsgProp goes to the follower, it could batch them and
forward to the leader. This can avoid dropping MsgProp in good path
due to exceed maximal serving in sender.

Moreover, batching MsgProp can increase the throughput of proposals that
come from follower.
Merge pull request #1901 from yichengq/260

rafthttp: batch MsgProp