For now we set the FileSource for the Worker context
but in the future after fixing #1664 we can assert()
that FileSource are set only for the Map thread context.
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.
Even though we only bind the shared_ptr in the after callback, the
worker thread was still copying the callback and therefore gaining
a shared reference. Here we try to eliminate the copies.
