Return search snippets

In v1, Sonic had no way of returning search snippets —portions of text in objects matching the search query. Its index was minimal, partly because original texts were never stored.

In Sonic v2, we will change that, and add the possibility to store original text in the index so matched portions can be returned in search queries.

Now that we’ve reached our target of supporting Terabyte-scale ingestion in ~6h (see #401), I’m moving on to implementing the optional storage of originals, and scoring of snippets.

Feel free to ask questions in this tracking issue, where I’ll be sharing my progress.