mds: Create MDSUtility as base for Dumper/Resetter

These classes has lots of similar functionality and we may
want to add more similar things in the future.

Also, get a copy of the MDS map at startup
to learn the ID of the metadata pool instead of
assuming it's the default.

Signed-off-by: John Spray <john.spray@inktank.com>
mds: Add dump-journal-entries

This adds a somewhat-human-readable peer to
the 'dump-journal' command.

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