prefetcher: address some review comments about rescheduling
prefetcher: address some review comments about rescheduling

Suggested by jzila.

Issue: #1915

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