client: enable using external Objecter

client: enable using external Objecter

To enable places that already have an objecter
(such as ceph-mgr) to avoid spinning up another
one inside Client.

This will also be a path to re-using the librados
objecter in multiple instances of libcephfs in the
same process, a la nfs-ganesha.

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