Migrate from Hive

[Bringoff]

So, hive seems to be taking rough turn with rewriting it using Rust and breaking backward compatibility. Also, it's not the most reliable database. It would be nice to migrate somewhere. Some candidates:

• ObjectBox — definitely a good option. I've been using GreenDao from same developers, and it was strong SQLite ORM. It makes some money by providing database server sync options, and not only for Flutter so it should be stable and mature. But it requires code generation. Not sure if we want this in the library.
• Sembast — popular NoSQL solution, similar to Hive, but it can be used on top of SQLite, so that's a plus. Also, I don't see plans from developer to rewrite it.
• Moor — nice test coverage, also sponsored by a company, so we should not worry about future of the library. But, again, it uses code generation. cc @algodave

[algodave]

@Bringoff Thank you for your proposals!

I'd suggest to keep it simple and avoid code generation. Sembast looks like a good candidate: it's a key-value store, similar to Hive. I'd definitely avoid its usage on top of sqflite because we don't want to force apps using this package to also use sqflite.

We could migrate easily from Hive to Sembast, and keep the filtering logic in-memory, just like we do now.