mon: shut down perfcounters last

In particular, remove_all_sessions updates the perfcounter stats, so we
need to stop those after.

Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #2022 from ceph/wip-mon-perf

mon: shut down perfcounters last

Reviewed-by: Joao Eduardo Luis <joao.luis@inktank.com>