mds: Remove some unused #defines

Signed-off-by: John Spray <john.spray@inktank.com>
mds/CDir: Remove a few lines of cruft

Signed-off-by: John Spray <john.spray@inktank.com>
mds: Don't create the /.ceph directory

This was entirely unused.  On existing filesystems
it will live on, on new ones it will never be created.

Signed-off-by: John Spray <john.spray@inktank.com>
mds: Add encode/decode/dump for use with dencoder

This is for the MDSTable subclasses: AnchorServer,
SnapServer, InoTable.  AnchorServer was partially there
but didn't work in practice dumped objects because of
the version prefix on the object.

Signed-off-by: John Spray <john.spray@inktank.com>