Instead of transforming between return value and callback, pass
the wrapped callback on to the invoked function as the last argument.
This eliminates the need for multiple invokeWithArgument overloads
and permits invoked functions to be asynchronous.
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.
