Motivation:
There's no way to override the default settings of the DnsNameResolvers
created by DnsNameResolverGroup because DnsNameResolverGroup is final.
Modifications:
- Make DnsNameResolverGroup non-final
- Add a new overridable protected method 'newResolver()' so that a user
can override it to create an alternative DnsNameResolver instance or
set the non-default properties
Result:
A user can configure the DnsNameResolver.
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