mds: add atomic log rewrite on format change

mds: add atomic log rewrite on format change

Two main pieces to this:
 * A new JournalPointer object that stores two journal
   inodes so that we can do a double-buffered update,
   followed by an atomic swap.
 * An extended recovery process in MDLog that dereferences
   the JournalPointer and conditionally rewrites the
   journal to accomodate format updates.

The JournalPointer indirection should also be useful for
making cephfs-journal-tool do updates more safely.

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