migrate backend to sqlite?

[c-cube]

I'm thinking of moving from having stateful plugins being able to store stuff in JSON, towards having them being able to use sqlite3 (the OCaml bindings). The advantages are numerous but in particular, having safe persistence and atomic operations would be a big plus for data integrity.

The existing default plugins would get their own table each (and plugins in general could declare their own table/indices at loading time). I'd also provide a little tool to go from json to sqlite3 for them.

Feedback welcome.

edit: see #41 for the corresponding issue.