Workaround for Race Condition in Issue #123

Solves https://github.com/Astron/Astron/issues/123.

Stops clients from disconnecting for trying to update on object that was visible but is no longer visible.
It may be the case that these "historical objects" should be cleared after a reasonable timeout (say 1 minute).
Additionally, it might be reasonable to resend the OBJECT_LEAVING message after receiving an update on a historical object.