Resume backup: JPA and ZIP archivers to use AEAbstractArchiver's _fopen for resumable file opens
Resume backup: Store file offset of each part file in the class in AEAbstractArchiver::_fwrite()
Resume backup: AEAbstractArchiver::_fopen() Seek to last position when re-opening file
Resume backup: AEAbstractArchiver::_removeFromOffsetsList() Remove the resume pointer from the class when creating a new part file
Do not repeat step number 1 in the log
Resume backup: Kettenrad reports the step number which has just finished and the step object's state (error, init, prepared, running, postrun, finished)

Also make sure there's the right number of Starting and Finished Step number log records
Resume backups: Add options akeeba.advanced.autoresume, akeeba.advanced.autoresume_timeout, akeeba.advanced.autoresume_maxretries to 05.tuning.ini
Resume backups: missing scale in options
AEAbstractArchiver: hold file pointers to the archive file to reduce disk I/O
AEArchiverJpa: reduce disk I/O by not opening and closing the archive all the time
AEArchiverZip: reduce disk I/O by not opening and closing the archive all the time
Resume backup: Auto-resume grew the archive bigger than it should