Add TransactionSetup and TransactionLimiter.

Add TransactionSetup and TransactionLimiter.

TransactionSetup allows a setup function to be called when making
transactions.  This allows transaction hooks to be installed.

TransactionLimiter provides a TransactionSetup that can be used
to limit the number of changes in a zone when loading via a file
or zone transfer.

Thanks to github users SaidBySolo and Mohammed Adnan for reporting
issues with unbounded zone size if the zone source was not trusted.