bookmarks: actual fix for race condition deleting bookmark
This is a simple but efficient fix to prevent the issue tested in
`test-bookmarks-corner-case.t`. It might be worth pursuing a more generic
approach where filecache learn to depend on each other, but that would not be
suitable for stable.
The issue is complicated enough that I documented the race and its current
solution as inline comment. See this comment for details on the fix.
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