Make _cmp generic for all EDNS Options. It now compares the wire formats

Make _cmp generic for all EDNS Options.  It now compares the wire formats
of two objects of the same class.

Previously we did type-specific stuff for ECSOption, but we only
checked the padded address info, not the srclen or scopelen, so
different options could compare the same.  Also, it's not clear that
there's any better semantic ordering of options than the wire format,
so we will just go for simplicity and use the generic implementation.