Fix races on shutdown in Muninn

The Muninn eviction thread did not anticipate that closing the page cache would
set all the MuninnPage references in the `pages` array to `null`, and so it
could die with a NullPointerException.

It now interprets the precense of a `null` value as a signal to shut down.
Merge pull request #2876 from davidegrohmann/2.2-fix-muninn-shutdown-races

Fix races on shutdown in Muninn