This is a follow-up of commit 2ca7c335,
which changed 'tweak' not to discard readable cells too quickly.

The scenario of a storage being lost whereas it has feeding cells was forgotten.
These must be discarded immediately, otherwise we end up with more up-to-date
cells than wanted. Without the change in outdate(), testSafeTweak would end
with: UU.|U.U|UUU

Once replication is optimized not to always restart checking cells from the
beginning:
- Remembering that an out-of-date cell was feeding could be a safer
  option, but it may not be worth the extra complexity.
- Another possibility may be to replace the FEEDING state by an automatic
  partial tweak that only discards up-to-date cells too many whenever a cell
  becomes up-to-date.