Alternative service implementations

[jnioche]

The default implementation uses RocksDB but it isn't meant to be a definitive, one-and-only solution. In fact, an important aspect of URLFrontier is that is an API and having competition for compatible implementations is encouraged. Of course, we also want the default one to be solid and reasonably efficient.

Below is a list of known implementation of the URLFrontier Service.

• OpenSearch Frontier provided by Presearch and usnig Opensearch as a backend.