Divest `hickory-resolver` library

Trippy is a traditional-style async network application in that it uses non-blocking IO and the select libc function.

The hickory-resolver library, used by trippy-dns to perform reverse DNS lookup (lookup TXT records), uses Rust async/await and provides a sync facade via the usual block_on pattern.

This forces Trippy to bundle an async runtime (tokio) which adds complexity and significantly increases the size of the executable.

Hickory is an excellent (best-in-class) library, but it is isn't suitable for use in Trippy.