Change base location for the LevelDB directory

[DiogoAndre]

Proposing a change in the location where the LevelDB files are stored on iOS.

Today the base directory is Documents/ and I'm proposing we change it to Library/Application support

Fixes #843

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[DiogoAndre]

Updated CLA

[wilhuff]

Thanks for bringing this up and thanks for the PR! I'll discuss this with the team today.

A problem with this in its current form is that it causes users with Firestore on a device to lose their data. In particular any writes pending synchronization would be lost.

The way to handle this and make things better going forward would be move any existing database from Documents to Application Support but that's not as easy to do as this small change, in part because our tests rely on being able to specify the directory that leveldb will use.

The trouble is we've set things up to delay all real work into start to avoid blocking the main thread during startup but we specify which directory to use to the initializer. This will need some rearranging to make it possible for start to consider both the legacy location and the new.

[morganchen12]

Closing this since this was resolved in #4609.