`migrateIfNeeded()` and multiple processes about sqliter

[kpgalligan]

I took a quick look at this and want to dig a bit deeper, so reopening for now.

[andersio]

I reckon iOS File Coordination can be used on the database files to provide guarantees that I need, without needing any change in SQLiter. Thanks anyway.

[andersio]

To add a bit of context on what I did: File Coordination is applied on the parent directory of the DB (in an App Group container), which makes sure no more than one process can run the create-or-migrate phase at the same time. This was done basically because we start developing app extensions, and they are spawned as throwaway processes by iOS.

The setup uses coordinateWritingItemAtURL with NSFileCoordinatorWritingForMerging, inspired by the recommended practices of a popular Swift SQLite library.