Stop raftNode goroutine before stopping server goroutine, so
server.Stop does stop all underlying stuffs elegantly now. This fixes
the problem that previous-round lock on WAL may not be released when
etcd is restarted.
Do not aggressively compact raft log entries. After a snapshot,
etcd server can compact the raft log upto snapshot index. etcd server
compacts to an index smaller than snapshot to keep some entries in memory.
The leader can still read out the in memory entries to send to a slightly
slow follower. If all the entries are compacted, the leader will send the
whole snapshot or read entries from disk if possible.
