dnsdist: Share tickets key between identical frontends created via YAML

dnsdist: Share tickets key between identical frontends created via YAML

Using the same Session Ticket Encryption Key on identical frontends
allow TLS sessions to be resumed in a much more efficient way, reducing
the latency and CPU usage. While it was already possible to do so by
manually managing the STEK, the default behaviour was to create and use
a different STEK for each frontend, because our Lua configuration makes
it almost impossible to ensure that two frontends are identical.
This is not an issue with the new YAML configuration format, so let's
share the STEK automatically in this case.

This needs a regression test.