Migrate to sqlitemap

[eigenein]

Ideas:

• shared collection for shared keys
• user_12345 collection for user-specific keys

Keep in mind, currently value is a string while in sqlitemap it's bytes. Possible migration:

create table "shared" (`key` text not null primary key, `value` blob not null);
insert into "shared" (`key`, value) select `key`, value from `default`;