Comment 2 for bug 1218503

This is apparently more serious than it first appeared.

- Looking at a VM 8.2.1 file, such as test-mime, is not only giving tons of Bad VM cache messages, but it is also crating duplicate copies of messages in the folder (which are then handled by the delete-duplicate-messages hook and getting flagged for deletion immediately).

- The fixed version of the code is not doing the right thing either if we insist on the length of the cache vector being = rather than >=.

This needs a closer investigation.