msg: allow calling dtor immediately after ctor

Asserting on reaper_stop only made sense if the
messenger had ever been started: as it stood,
one couldn't create and destroy a messenger
without also starting and stopping it.

Signed-off-by: John Spray <john.spray@redhat.com>
Merge pull request #2574 from ceph/wip-msgr-shutdown

msg: allow calling dtor immediately after ctor

Reviewed-by: Sage Weil <sage@redhat.com>