Event Pipeline

Create a pipeline to pass events through. All events we encounter, through the firehose, the relay pool, the Ditto relay, and generated by Mastodon API controllers, are passed through the pipeline.

The pipeline stores all events it encounters from local users. To "post" an event, it just needs to go through the pipeline. It tracks relays and hashtags from events from any source. Later it will integrate with strfry-policies to provide MRF-like filtering.

The original "publish" function has been removed, since now "publishing" an event just means making it available on the Ditto relay. But later we will federate events to the relays of other users with a gossip table.