[Sheepdog] [PATCH 0/4] some optimization for cluster recovery.

[Li Wenpeng]

In this patch set, I did the following work:

1. By only writing objects no longer belong to the current node 
   to the snapshot, avoid unnecessary IO operation in farm when cluster
   is recovering,
2. Add a nr_zones field to cluster_info, which is maintained by sheep,
   so we can use it anywhere instantly, instead of recalculating it.
3. Add a cleanup operation for collie for users to clean up the useless
   trunk files in farm.
4. No longer store the trunk map in file, instead, I put it in memory
   using rb-tree, after restart, we just need to clean up the old trunk
   files by the cleanup function implemented in patch 3.