Support for staged PFFiles in Local Datastore

[nlutsenko]

Even though you can pin PFObjects to Local Datastore, all unsaved PFFiles are not persisted across runtimes of the application.
This issue is for tracking/discussing potential support for this as well as tracking implementation progress.

There are multiple reasons why this is not been implemented so far, including:

• The only unique identifier that a PFFile has is a url, that is generated only after the file is saved.
• Tracking object references to PFFiles in Local Datastore is tricky and gets even more tricky when the files should be deleted on object removal from LDS.

The rough ideas:

• Move PFFile staging outside of Caches directory (so we have a guarantee on persistence)
• When PFFile is staged - add a custom field to the file that would store a generated UUID
• This UUID would also be used for file name, so we can reconstruct a path for reading/saving
• Track PFFile references in Local Datastore, and at the moment where there are no more files - delete the actual contents of the staged files.

cc @grantland @hallucinogen @stanleyw

[FreudGit]

+1, sounds great. :)

[FreudGit]

@nlutsenko Any news on this?

[stale]

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