Fix bug in shouldRetry with closed linked cursor

When we discover, via shouldretry, that a cursor needs to retry, the retry will
cascade to all linked cursors, if any.
If the cursor has previously had a linked cursor, but the linked cursor was
closed, then the parent cursor is not notified and will still traverse through
the closed linked cursor when starting retry.
This would cause the startRetry to throw a NullPointerException, because
a closed cursor drops its reference to the MuninnPagedFile.
The bug is fixed by checking that the pinned page ref is still valid, which is
the standard way of telling whether a cursor has been closed or not.
This issue was discovered by GBPTreeConcurrencyIT, which deterministically
failed.
PageCacheTests have been added in a different PR because I wanted them to go
into 3.2, instead of 3.3.

community/io/src/main/java/org/neo4j/io/pagecache/impl/muninn/MuninnReadPageCursor.java

@@ -117,7 +117,10 @@ private void startRetryLinkedChain() throws IOException
         MuninnReadPageCursor cursor = this;
         do
         {
-            cursor.startRetry();
+            if ( cursor.pinnedPageRef != 0 )
+            {
+                cursor.startRetry();
+            }
             cursor = (MuninnReadPageCursor) cursor.linkedCursor;
         }
         while ( cursor != null );