etcdserver: not load dummy entry from the wal
etcdserver: remove finished TODO for raftStorage.Compact
etcdserver: init snapi when init appliedi
etcdserver: panic when snapshot on raft storage

Snapshot on raft storage should always succeed. If there is an error, it must
be internal fault and needs stack info to debug.
raft: add comment to newLog
raft: remove useless line in raftLog.append
raft: simplify raftLog.lastIndex
raft: refine raftLog.term
raft: not set applied when restore log from snapshot

applied is only updated by application level through Advance.
raft: remove unused raftLog.at function
raft: stricter checking in raftLog.slice
raft: remove unused raftLog.isOutOfAppliedBounds
raft: remove wont-fix TODO in ApplyConfChange
raft: set snapshot to nil when it is saved
raft: use IsEmptySnap to check the empty snapshot
raft: make if checking match the error in storage.Term