mds: Fix crash on "mds stop" of rank 0

Previously could reliably hit assertion by
doing "set max_mds 0" then "mds stop 0".  The
Journaler was trying to use the Objecter after
it had been shut down by suicide().

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>
HACKHACKHACK printing formatted LogEvents