rafthttp: add batcher
After we enable streaming, there will be a large amount of msgAppResp due to high
rate msgApp. We should batch msgAppResp in a meanful way.
Merge pull request #1768 from xiang90/batcher

rafthttp: add batcher
Merge remote-tracking branch 'upstream/master'

* upstream/master:
  rafthttp: add batcher After we enable streaming, there will be a large amount of msgAppResp due to high rate msgApp. We should batch msgAppResp in a meanful way.