prefetcher: key queued requests by ptr, not by block ID
prefetcher: key queued requests by ptr, not by block ID

If we use block ID, we might end up canceling legitimate requests for
deduplicated blocks with the same ID but a different nonce as the one
being canceled.

Issue: KBFS-3756

keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client