mbgl::Thread will keep a ThreadContext for each running instance
in a thread_local so we don't need to lookup a man in the Environment
every time we need some info about the current thread.
This patch is moving the ::currentlyOn check used on Debug build from
the Environment class to the ThreadContext.
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.
