Use new block for pre-RollingUpgrade files' append requests

After the RollingUpgrade prepare, append on pre-RU files will re-open the same last block and makes changes to it (appending extra data, changing genstamp etc).
These changes to the block will not be tracked in Datanodes (either in trash or via hardlinks)

This creates problem if RollingUpgrade.Rollback is called.
Since block state and size both changed, after rollback block will be marked corrupted.

To avoid this, first time append on pre-RU files can be forced to write to new block itself.