Reworking TTMs LRU handling

[Christian König]

Hi guys,

in the last set Felix pointed out a severe bug in the iterator
implementation and after investigating I've found that my test case
didn't even exercised this code path.

So after fixing the test case I've found a couple of more minor
problems. This is the resulting patch set with everything fixed as far
as I can see.

Please review and/or comment,
Chriustian.