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
line 159 needs to go through not only when there is a proposal.
The problems is that MsgProp is not guanranteed to come continuously. For example, if we batch one proposal, and no more proposals come, we want it to be sent out when timeout instead of batched.