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.
(cherry picked from commit 848094d6)
Closes #3129 (closed)