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.