uridownloader: make cancelled state 'permanent' until a reset

uridownloader: make cancelled state 'permanent' until a reset

When downloading and cancelling quickly the uridownloader object and the
element using it could miss the cancelled window and the uridownloader
would fetch the wrong URI and block on subsequent fetches.

This was also problematic when stopping elements, while one task would
call the cancel, another element thread could issue a new fetch_uri. As
the cancel state isn't 'permanent' this fetch_uri would block and
prevent the whole element from stopping and going to NULL.

This patch makes the 'cancelled' state permanent until a
gst_uri_downloader_reset is called. This way the element knows the
window where the uridownloader isn't active and only reactivate it when
ready.