Though this is a revert, it also involves fixing the code with new code
and a regression test. The problematic locking that was there before has
been removed, in any case.
This reverts commit ba728e46be.
Reason for revert: regression.
Fixes: https://git.lix.systems/lix-project/lix/issues/647
Change-Id: I326b1a10042ff69afbab38426e042e0dc2224a13
