client/authority-discovery: Split into worker and service mod

Move `Service` and `Worker` to their own Rust modules resulting in the
following file structure.

├── build.rs
├── Cargo.toml
└── src
    ├── error.rs
    ├── lib.rs
    ├── service.rs
    ├── tests.rs
    ├── worker
    │   ├── addr_cache.rs
    │   ├── schema
    │   │   └── dht.proto
    │   └── tests.rs
    └── worker.rs