Arvados

In the configuration used by crunch-job, arv-mount --mount-by-pdh=by_pdh --mount-tmp=tmp subscribes to websockets even though it is unnecessary and not intended.

Each *Directory object (various classes in fusedir.py) should have a method that reports whether it needs websocket updates. After/while setting up mounts in command.py, ask all of the dir objects whether any of them need events; if not, don't subscribe.