rafthttp: increase sender buffer size

The buffer size is set big enough to buffer all messages generated in
one second as a follower in good path.

In good path, a follower will only send MsgAppResp back. Considering our peak throughput is around 4000req/s, and the batch number is 100, it is good when we can buffer 40 messages.

fixes #1787 
both are fine.
updated.