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
pkg/transport: add timeout dailer and timeout listener
integration: longer request timeout for slow travis
Merge pull request #1771 from xiang90/listener

pkg/transport: add timeout dailer and timeout listener
rafthttp: log start and stop of streaming
pkg/transport: fix dialer typo
*: set read/write timeout for raft transport and listener