prefetcher: use the previous request priority for sync fetches
prefetcher: use the previous request priority for sync fetches

And just subtract 1 for the priority of each triggered level that's
not a sync fetch.

Suggested by jzila.

Issue: #1932

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