Fix an ABA bug in the page cache

We were reusing the MuninnPage objects in the MuninnPageCache.freelist.
This could lead to cycles in the freelist through an ABA-bug.

The test that exposed that bug is ignored for now, because it is exposing
another bug that I thought was just a different manifestation of the ABA-bug.
Merge pull request #3247 from chrisvest/2.2-pagecache-freelist-aba

[2.2] Fix an ABA bug in the page cache