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>