Add a basic `AuthorityDiscoveryService` implementation which enables
callers to get the addresses for a given `AuthorityId` from the local
cache.

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

Instead of updating the authority discovery peerset priority group each
time a new DHT value is found, update it regularly on an interval.

This removes the need for deterministic random selection. Instead of
trying to return a random stable set of `Multiaddr`s, the `AddrCache`
now returns a random set on each call.