Fixes #8538 - ISC dhcp plugin now respets binding

Good catch. Hmm, `find` only returns the first occurance, this will not work. As I just learned, there is a helper method `kind` that compares records by it's class. I've used this approach, added your example to the test case. Thanks.

One day, I will rewrite DHCP from scratch. I have couple of ideas how to make this fast.