Support directory lock

[yvxiang]

• Store the lock mark in FileInfo
• Store the lock mark in namespace, and remove the mark when the lock is released. Keep a copy of lock list in memory.
• Rebuild the lock list when Nameserver restarts. For each locked dir, add all the files under this dir to incomplete list. (Nameserver restarts does not change the dir locking status)
• Regarding HA: the lock mark should send to slaves/followers via Sync
• API should includes LockDir and UnlockDir
• 在FileInfo中持久化locked标记
• 给目录加锁时将namespace中对应目录的标记位置位，解决锁时将对应标记位清除，同时内存中维护一个已加锁的目录列表
• nameserver重启时，Rebuild过程中发现该目录被锁，则scan时，将其下所有文件的block扔到incomplete列表中，将对应目录的状态恢复为已加锁（即name server重启时，并不掉锁）
• HA时的处理：lock unlock都是写namespace，所以天然持久化，若发生切主，则同nameserver重启走相同流程
• 结构：LockDir和UnlockDir