mds: expose progress during PurgeQueue drain

mds: expose progress during PurgeQueue drain

We don't track an item count, but we do have
a number of bytes left in the Journaler, so
can use that to give an indication of progress
while the MDS rank shutdown is waiting for
the PurgeQueue to do its thing.

Also lift the ops limit on the PurgeQueue
when it goes into the drain phase.

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