mds: shut down finisher before objecter

mds: shut down finisher before objecter

Some of the finisher contexts would try to call into Objecter.
We mostly are protected from this by mds_lock+the stopping
flag, but at the Filer level there's no mds_lock, so in the
case of file size probing we have a problem.

Fixes: http://tracker.ceph.com/issues/19204
Signed-off-by: John Spray <john.spray@redhat.com>
(cherry picked from commit 177a97d5c55ee6a2d5dcd3cf0893546190b10f7a)

Conflicts:
        src/mds/MDSRank.cc (no purge_queue.shutdown() in jewel because jewel
        does not have 8ebf7d95a9071de24bb1e56a6423c505169cb4de)