Memory leak appearing since 6.0.9

Some deployments suffer from this and just reverting to 6.0.8 fixes that. The behavior is distinct from #873

Example report: https://lists.nic.cz/hyperkitty/list/knot-resolver-users@lists.nic.cz/message/4LSLNYHA5TR3QXSLDILKJBJCS47QWDTO/

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information