folder_block_ops: add a way to fetch generic blocks in parallel
folder_block_ops: add a way to fetch generic blocks in parallel

Issue: KBFS-1777

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