Expirations / MBR liquidation

In order for someone to become the new owner of an NFD once it has expired, all metadata must first be cleared. This is similar to listing an NFD for sale, where all metadata must first be cleared.

An exception with expired NFDs, is that ANYONE can clear the metadata of an NFD if its expired. Anything deleting the fields of an NFD via the new deleteFields ABI call, will receive the MBR held by the box storage being freed. The same applies to calling the registry to unlinkNfdAddress method - returning the box storage MBR if no data remains once unlinking.

This allows a type of 'liquidation' feature where on-chain NFD data can be cleared once the NFD expires by providing an incentive for anyone to take advantage of.

If an NFD isn't cleared, then the UI adds deleteField calls prior to the renew so that the new owner is able to claim the expired NFD.