folder_branch_ops: decrease outstanding write sizes
folder_branch_ops: decrease outstanding write sizes

To deal with timeouts on slow networks, try to limit the amount of
outstanding write data to the block size * the number of maximum
blocks that can be put in parallel.

Issue: KBFS-682

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