Revert "Allow image sources to specify schedulers to subscribe on"
This reverts commit bb48bb6c9f57df342da21ebc12b2c421a54ea2ee.

This was a bad idea after all—by Rx guidelines, Subscribe is not supposed to do any blocking work.
Moreover, it makes immediate cancellation impossible.
Implement basic cancellation so that if cached file is found, no request is made. Move actual work to TPL tasks.