mon: ceph-monstore-tool must close()

Otherwise it asserts because the destructor checks for the is_open flag.

http://tracker.ceph.com/issues/10093 Fixes: #10093

Signed-off-by: Loic Dachary <ldachary@redhat.com>
Merge pull request #2914 from dachary/wip-10093-monstore-tool

mon: ceph-monstore-tool must close()

Reviewed-by: Joao Eduardo Luis <joao@redhat.com>