The util::Thread will call the stop() method of the MapContext's RunLoop
which will wait for the pending tasks tied to it to complete.
If we have a request that is timeout'ing, this could mean a really long
wait. Instead, we now send a cleanup message that will reset all the
attributes first (canceling the pending requests) so the the MapContext
gets destructed quickly.
The MapContext can only run on its own thread because it is
registering a EnvironmentScope and it is making many assertions
on Debug mode if its methods are being called on the Map thread.
