raftserver: use front_mut instead of pop first.
raftserver: use front_mut instead of pop first.

if we pop first and write failed, we can retry writing this buffer
never.

BusyJay/tikv,BusyJay/tikv,c4pt0r/tikv,BusyJay/tikv,pingcap/tikv,pingcap/tikv,BusyJay/tikv