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.
we are now scoping all file requests to an environment object. The FileSource implementation treats
this as an opaque pointer, but allows canceling all Requests that are associated with that pointer.
This is necessary to abort all file requests that originated from a particular Map object. Aborting
a file request is different from canceling a file request: A canceled request doesn't have its
callback called, while an aborted request will have its callback called with an error, indicating
that the environment is going to be shut down.
