Improve zeroed blockalignck skipping during full image backup
Improve zeroed blockalignck skipping during full image backup

(cherry picked from commit 17e5b5ed8b4d88cd2e40d3019bb016dd9009609a)

uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend