Release SyncFileItem objects with their job

Release SyncFileItem objects with their job

We now delete subjobs as their propagation is complete. This allows us
to also release the item by making sure that nothing else is holding a
reference to it.

Remove the stored SyncFileItemVector from SyncEngine and SyncResult
and instead gather the needed info progressively as each itemCompleted
signal is emitted.

This frees some holes on the heap as propagation goes, allowing many
memory allocations without the need of requesting more virtual memory
from the OS, preventing the memory usage from increasingly growing.