The previous implementation, based on thread-local storage, did not
ensure that the context was destructed before the FileSource run loop.
This resulted in implementations attempting to uv_close handles for a
loop that had already been destroyed.
This change also fixes#1262.
SharedRequestBase was trying to do two different jobs: be a base class
for Asset/HttpRequest and track internal state for DefaultFileSource.
Now we have RequestBase for the former, and DefaultFileRequest for the
latter.
