Remove sqflite

[781flyingdutchman]

The plugin adds substantially to the size of the package, and is only used by those who need SqLite functionality.

We can move classes like SqlitePersistentStorage to the example app, so they can still be used by copying them into the developer application.

Planned for V8

[781flyingdutchman]

In V8, moved all SQLIte related code to a separate package (not yet published at the time of this comment, as it requires V8 to actually be published first!). Add background_downloader_sql to your dependencies in pubspec.yaml to get SqlitePersistentStorage and SQLite related migration options back.

The reason for this change is that the sqflite dependency adds significant size to apps, even if they do not use the SQLite functionality.

[markst]

Thanks for this!