Avoid zero-payload backups in local transport

The local debugging transport now implements
BackupTransport.checkFullBackupSize() to detect and reject backup attempts
for which no actual file content will be committed.  The documentation for
checkFullBackupSize() has also been expanded to document the transport's
responsibilities in this regard.

The local transport now lazy-creates the destination file when data is
first delivered for an approved backup operation, rather than doing it
proactively in performBackup(), to ensure that changes in the datastore
are only attempted after the transport has positive confirmation that
data is indeed flowing.

Change-Id: I6e47a7e72cd938fc0ed31da4bc490540c71f9e65