Add sqlite cache

[jkmassel]

Adds an initial SQLite cache implementation that works on Android and iOS, implements row-level change updates, and has a migration mechanism.

We'll need the ability to store and fetch objects, but a lot of this should just be generated code instead of being hand-written – that's for a future PR.

[crazytonyli]

If the database is designed to be used as a cache, do we even need to migrate the data? By definition, users should not expect cached data always to be there, right?

[jkmassel]

If the database is designed to be used as a cache, do we even need to migrate the data

This is a great question. I see this as a replacement for Core Data / FluxC – so while we could just throw away the data every time we need to change something, it seems wasteful of system resources. For a large site, destroying the cache could mean re-downloading thousands of posts/media items/comments/etc. That's a lot of CPU time and battery that we don't need to spend if we're a little bit careful about our data model.

I also like this model of migrations because each one can be small and easy to reason about – if something weird happens you'll know where it happened in the chain and can pick up where you left off.

Though this is still a cache – if something goes wrong with it, the move is to destroy it and re-create it, so we don't need to be quite as careful as user data.

I think this PR represents a good balance between flexibility and being careful about user resources, but happy to discuss further if anyone disagrees!

[crazytonyli]

For a large site, destroying the cache could mean re-downloading thousands of posts/media items/comments/etc.

I think the "re-downloading" is going to happen regardless of whether the posts have been fetched and saved in the local SQLite database. When users go to view the post list, we'd have to re-download every post again. That's the only way we can guarantee that the most recent versions of the posts are displayed on the list.

[oguzkocer]

@jkmassel Thanks for the POC! I've left some suggestions.

[oguzkocer]

I've mentioned this on Slack as well, but just leaving a note for myself. I've re-reviewed up to 0bedb9c and it's looking great. Just a couple more unaddressed comments and I think we are good to merge this.

[oguzkocer]

I believe all the issues in Rust are addressed. I am approving it so we can merge it once the Swift issues are resolved.