Merge #147

Merge #147

147: Cancelation r=matklad a=matklad

This series of commits switch cancellation strategy from `JobToken` (which are cancellation tokens, explicitly controlled by the called) to salsa built-in auto cancellation. "Auto" means that, as soon as we advance the revision, all pending queries are cancelled automatically, and this looks like a semantic we actually want.

"client-side" cancellation is a rare event, and it's ok to just punt on it. Automatic cancellation after the user types something in happens all the time.

Co-authored-by: Aleksey Kladov <aleksey.kladov@gmail.com>