I noticed this while working on WCFS: if file blocks topology change,
the invalidation process is not working correctly. It is also not
correct with respect to live cache pressure.

Add FIXME in the code and test for live cache pressure.

kirr/wendelin.core@5a4562fc
kirr/wendelin.core@48eb692f
kirr/wendelin.core@d1a579b2
kirr/wendelin.core@69c94fbc