folder_branch_ops: don't let background tasks get stuck forever
folder_branch_ops: don't let background tasks get stuck forever

If there's a bug, or something weird happens on the network, we don't
want the whole file system to freeze forever.

Issue: KBFS-683

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,keybase/client