Slab rebalancer corner cases

Issue to track rebalancer corner cases as discovered while solving #1239 and #1248

• If the page mover is moving from a class with nonzero chunks in use and zero actual items, it cannot evict to make progress.
• This happens if a class is full of only chunked item caps.
• In general the new page mover needs a backstop to abort if it's hung for a long time, so it can potentially be told to try again from a different class.
• The memory-mode automover needs to take into account chunked-cap-only classes, recognize those as being unbalanced, and force move a page into them.

These extra corner cases require more code than I want to add to the next release for too little benefit, so I'm opening this issue to keep track of them for the next release.