Suggestion: Make teardown of filesystems idempotent

[mulkieran]

The motivation for this is that if a filesystem ends up torn down, somehow, and stratisd does not detect that, then the pool will be wedged and can not be torn down, because an entry for the filesystem remains up and an error is returned when an attempt to tear down the filesystem is made.

The work around is to restart stratisd, or to use the stratis debug refresh command. In many cases, that should bring the filesystem back up, and restore an ability to stop it.