This reverts commit fb265b610d.
The optimization in that commit is safe when the executor thread is
writing and the GUI thread is reading, but I had failed to take into
account that it's unsafe when the GUI thread is writing and the executor
thread is reading. (The native UpdateAdditionalMetadata function loops
through m_cached_files, which is unsafe if another thread is adding
elements to m_cached_files simultaneously.)
Losing out on this optimization isn't too bad, because
719930bb39 makes it very unlikely that
both threads will want the lock at the same time.
51debaeb47