client: send metadata in session open

Populated with hostname and entity_id by
default, with interface for outer layers
like ceph_fuse, libcephfs to inject their
own metadata.

Signed-off-by: John Spray <john.spray@redhat.com>
ceph_fuse: include mount point in client metadata

Signed-off-by: John Spray <john.spray@redhat.com>
mds: generate friendly name for client sessions

...from the client metadata, for use in health messages.

Signed-off-by: John Spray <john.spray@redhat.com>
mds: implement const SessionMap::get_session

Signed-off-by: John Spray <john.spray@redhat.com>
mds: use new Session::human_name in health msgs

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