want method to retrieve bootstrap resolvers by domain name

[kusor]

When using resolverForIpOrDomain, domain resolvers are stored at class level into CueBallDNSResolver.bootstrapResolvers. Those in general are not needed but on the cases where the DNS server for the provided domain name is down. Then, the aforementioned DNSResolver will keep trying to connect forever unless process.exit is used.

It would be desirable to have a public API method to retrieve these resolvers using domain name, allowing applications using cueball to stop them when needed.