Implement TCP support in the `ans11` Python-based DNS server.
Implement a control command channel in `ans11` to support an optional
silent mode of operation, which, when enabled, will ignore incoming
queries.
In the added check, make the `ans11` the NS server of
"a.root-servers.nil." for `ns3`, so it uses `ans11` (in silent mode)
for the regular (non-forwarded) name resolutions.
This will trigger the "hung fetch" scenario, which was causing `named`
to crash.
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