Add ceph-client-debug inode lookup


The commit chain is a bit messy, can squash this before merging.

This became a little more complicated once I thought about the safety of the tool retrieving Inode objects after sending a lookup operation: to avoid the risk of the Inode being gone by the time the client gets around to retrieving it, the lookup_ino functions take a reference to the inode inside the client lock.  This is a bit invasive but seems to be necessary to avoid the calling code having to do a "get, got?" retry loop.  This is based on the assumption that the Client is free to delete inodes at any time.