MDEV-39209: use iterative cleanup for merged units to avoid stack overflow#4919

DaveGosselin-MariaDB wants to merge 2 commits into10.11MariaDB/server:10.11from

10.11-MDEV-39209-stranded-list-depthMariaDB/server:10.11-MDEV-39209-stranded-list-depthCopy head branch name to clipboard