Let the last thread dispose of pending references

Let the last thread dispose of pending references

If we're idle, there might be objects pending cleanup for the last two
epochs. Unless another thread comes in and checks into a new epoch or we
shut down, they will linger forever.

If one of the objects was a connection, it wouldn't get closed and be
stuck in CLOSE_WAIT state, potentially refusing another ligitimate
connection if its socket address were to match the one we're yet to
close.