This happens automatically in the background anyway, and it can take a
long time on low powered devices at an inconvenient time. We just want
to get up and running as quickly as possible.
GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3000
We're going to need the db.Instance to keep some state, and for that to
work we need the same one passed around everywhere. Hence this moves the
leveldb-specific file opening stuff into the db package and exports the
dbInstance type.
