addrmgr.GetAddress() had a parameter `class string` originally intended
to support looking up addresses according to some type of filter such as
IPv4, IPv6, and only those which support specific wire.ServiceFlags
(full nodes, nodes that support bloom filters, nodes that support
segwit, etc). But currently the parameter is unused and also has an
inappropriate type `string`.
If it would ever be used, it's easy to add back and should then get an
appropriate type such as something that allows bitflags to be set so
that the caller could request combinations such as peers that support
IPv6, are full nodes, and support bloom filters.
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