Added TSIGKeyPtr instance member to NameChangeTransaction.
Modified NameChangeTransaction::sendUpdate() to do a TSIG updates if its
TSIGKeyPtr is not null.

Extended FauxServer test class to support TSIG if given a key.
Added round trip TSIG tests to NameChangeTransaction base class tests.

This does not address how the transaction's key is determined. That
requires changes to configuration classes in D2Config.