mds: Fix Dumper shutdown

It was shutting down messenger before monclient.  This
caused a sometimes-assertion when monclient tried to
reconnect in response to ms_handle_reset.

Signed-off-by: John Spray <john.spray@inktank.com>
mds: Remove hardcoded pool ID in Dumper

Instead, get a copy of the MDS map at startup
to learn the ID of the metadata pool.

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

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>