This PR fixes copy-paste error in the product usage code where we were
taking out the authLock to access the mount table.
While we're add it we can remove the existing lock grabbing in the
product usage goroutine in favor of a serialized startup/teardown of
censusManager and its core dependency which requires the lock. This
requires some minor test edits, so created a test helper for that.
By moving the censusManager teardown before expirationManager teardown,
we can effectively ensure the goroutine is completely stopped outside of
any expirationManager change.
We are already guaranteed serial startup, so this should free us of any
complex lock semantics.
