Instead of validating rrsets and sections after parsing them, check the
class/type for each record before parsing it.  This is more generic,
because it moves all of the update logic out of the common code.  It's
also more flexible, as it allows the update logic to specify that
meta-records are empty.

Instead of parsing the header to determine which Message subclass to
create and passing that message to _WireReader, make _WireReader create
the Message subclass itself.