mds: cephfs-debug utility

Proof of concept providing:
 * Get JSON inode info by inode ID (backward lookup)
 * Get JSON inode info by path (forward lookup)
 * Scrape files out of data pool by path filter
mds: Remove unused MDS reference in MetaBlob
mds: If wanting ONESHOT_REPLAY, do nothing else

It was possible for the mon to put an MDS into a
different state than that which it asked for if
the MDS hadn't got the latest map at the time.