Port to FutureSQL

This reduces the manual work necessary to run the database on a separate thread, and also simplifies parsing the results.

Depends on libraries/futuresql@00f2cf84, which will be released over the next hours.

This still needs some more complete testing, which I can't do, so it would be nice if someone could make a backup of their database

cp ~/.local/share/spacebar/messages.sqlite ~/.local/share/spacebar/messages.sqlite.bak

and then run this merge request on a real database. If any queries go wrong, please send me the console output.