Make "Service" actor smarter

[schwa]

We shouldn't be accessing MastodonAPI for most stuff. The Service actor should have easy to use APIs that let you fetch data…

1. Cache all objects (Statuses, Accounts, Relationships being the core objects). Store them in Storage object.
2. Fetch objects if not in cache (storing individual objects and pages of objects if using a paging api).
3. Provide didChange mechanism (Swift Async Algorithm channels) so that UI can handle changes automatically.

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[schwa]

We have this partially working for timelines and relationships.