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, the batch can increase the throughput of proposals that come
from follower.

/cc @xiang90 @bdarnell
i think we should move the batching logic to etcdserver.