Implement a new Python class, AsyncDnsServer, which can be used by
ans.py scripts placed in ansX/ system test subdirectories. This enables
conveniently starting a feature-limited, non-standards-compliant, custom
DNS server instance. It can read and serve zone files, but it is also
able to evaluate any user-provided query-processing logic, allowing
query responses to be changed, delayed, or dropped altogether. These
are all actions commonly taken by custom DNS servers written in Python
that are used in BIND 9 system tests. Having a single "base"
implementation of such a custom DNS server reduces code duplication,
improving test maintainability.
Co-authored-by: Tom Krizek <tkrizek@isc.org>
