Added support for TSIG to D2UpdateMessage and DNSClient
Change D2UpdateMessage to support TSIG signing and verification via its toWire and fromWire methods. Both now accept a pointer to a TSIGContext, which they should use, if its not NULL. Implemented DNSCLient::doUpdate variant that accepts a TSIGKey. It will use the key to create a TSIGContext that will then be used to sign the outbound request and to verify the response in the operator() method. Added appropriate unit tests.
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