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.
Test whenever destroying a Map object with requests pending
works and is done quickly.
The test injects artificial delays in selected requests and
tries to destroy the Map object afterwards.
Currently glyph requests are skipped because there is a bug
being worked on in a different issue.
