[ios] Refactor MGLOfflineStorage

[julianrex]

In the context of working on #15536, it's become apparent that we should consider refactoring MGLOfflineStorage to include:

1. Improve testing by allocating a test specific MGLOfflineStorage, and not relying on sharedOfflineStorage: [ios, macos] remove references to [MGLOfflineStorage sharedOfflineStorage] from internal methods. #13877
2. Support accessing from a background thread: OfflineMapManager is crashing on serial queue access #15536 & [iOS] Offline download from background thread #14206
3. Improve progress notifications: MGLOfflineStorage addContentsOfURL with progress #14877
4. Support background downloads: Support background transfers on iOS #4291

[1ec5]

Support background downloads: #4291

Background downloads will require some significant refactoring in mbgl, not just the SDK code, because background download tasks break several of mbgl’s assumptions around offline downloads.

[julianrex]

Closing as won't-fix/stale. cc @knov