* to get rid of global state
* to have a testable architecture
* to allow for authenticated encryption
* to have a backup format version
* to potentially allow for other storage plugins
For the current transport it is important to know when the backup ends,
because it resets its state only then and closes the ZIP file.
The detection was broken,
because some packages didn't have data to back up (LOG_EVENT_ID_NO_DATA_TO_SEND),
so the transport's methods weren't called and the package counter not updated.
The hacky solution is to use the BackupObserver to call back into the
transport at the end of backup.
Ideally, future transports won't need to know when the backup finishes.
