This is branches/zip@r6032 in SVN and _is part_ of
revid:svn-v4:16c675df-0fcb-4bc9-8058-dcc011a37293:branches/zip:6113
in BZR.

This is being reverted because now the code is serialized directly on
index->stat_n_diff_key_vals[] as the fix for
Bug#53046 dict_update_statistics_low can still be run concurrently on same table
goes.