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I thought that explicit type name made the code more readable, but maybe
that's not the case so I've removed it now.

It still needs to be specified in the other constructor.

Instead, make the one that returns ConstRRsetPtr virtual.

the latter doesn't always work.

This commit also changes the base class's find() prototype to include
the RRClass as an argument. Otherwise, we don't know what RRClass to
look for. The arguments for the base class's find() method are in a
different order from those of the find() methods in the RRsetCollection
implementation as there's no way to overload the method when only its
return value is different.