rafthttp: only batch good MsgAppResp

A MsgAppResp with Reject set should be sent back to the leader as soon
as possible instead of batching.
Merge pull request #1789 from yichengq/234

rafthttp: only batch good MsgAppResp
version: bump to alpha.4
raft: always write dummy entry to storage
Merge pull request #1792 from xiang90/fix_raft_l

raft: always write dummy entry to storage
Merge branch 'log_interface'

Conflicts:
	raft/log.go