It is safe to repair the unexpectedEOF error in the last wal. raft
will not send out message before the entry successfully comitted
into wal. Thus we can safely truncate the last entry in the wal
to repair.
WAL should control the cut logic itself. We want to do falloc to
per allocate the space for a segmented wal file at the beginning
and cut it when it size reaches the limit.
Xiang Li