- obsim just stops publishing updates. Consumers may have their own
policy for dealing with this. e.g., if I don't hear about this obstacle
after 10 secs, I'll drop it.

- obsim tells the consumer: As long as this thing exists, I'll be sending you
updates every N seconds. Downstream apps still have a drop-policy, but the
drop-policy guidance comes with the obstacle

- obsim tells the consumer when to drop the obstacle. It will send a message
to downstream app: this object is done! Downstream app keeps it forever unless
it is told to drop it


Sim Action                       Consumer drop-policy
-----------                      ---------------------
send and forget

send and stale after N secs

send and stale when I explicitly
send another message and say it
is stale

send and consider it stale
if you haven't heard back from