1. Kill canceled instances; we allow users to "terminate" an instance
   while it is booting up, but we have to pend that till the lock is
   released. We do this with a canceled flag, similar to the Classic
   interface. But I never committed the apt_daemon changes that look for
   canceled instances and kills them!

2. Look for stale st/lt datasets and delete them. A stale dataset is one
   that no longer exists at the remote cluster (cause its expiration was
   reached and it was reaped). We do not get notification at the Portal,
   and so those dangling datasets descriptors sit around confusing
   people (okay, confusing me and others of a similar vintage).