Use isctest.asyncserver in the "chain" test

Use isctest.asyncserver in the "chain" test

Replace the custom DNS servers used in the "chain" system test with
new code based on the isctest.asyncserver module.

For ans3, replace the sequence of logical conditions present in Perl
code with zone files and a limited amount of custom logic applied on top
of them where necessary.

For ans4, replace the ctl_channel() and create_response() functions with
a custom control command handler coupled with a dynamically instantiated
response handler, making the code more robust and readable.

Migrate sendcmd() and its uses to the new way of sending control queries
to custom servers used in system tests.

(cherry picked from commit c3d3c9955d61489bc38a5c137d318c7333b2be9e)