External payload files are now stored in levelled folder hierarchy. Currently we
implement one level of indirection using modulo 100 of Part.id(). Using modulo
100 ensures even distribution of files into the subdirectories. The migration is
implemented in StorageJanitor, so it can be triggered manually by running
akonadictl fsck.

Handling of access to external files has been refactored to ExternalPartStorage
class. This class implements access to the legacy flat-cache hierarchy as well
as simple transactional system. The transactional system allows us to rollback
or commit changes in the external files: file created in the transaction are
deleted when the transaction is rolled back, and file deletion is delayed until
the transaction is committed. This allows us to control the files from outside
PartStreamer and tie it to committing of database transaction, which is more
likely to fail than the EPS transaction. This should prevent us from losing
cached parts when error occurs during update.

BUG: 338402
FIXED-IN: 15.12.0