Forced eviction tweak

@agorrod, this is the only idea I've had, and it isn't perfect.

It attempt to evict any page that hasn't yet been modified by the current transaction.  There are likely to be a lot of failures, because the page may contain recent updates that some running transaction needs to read.  OTOH, if I tighten up the forced eviction check to only try pages with globally visible changes, I don't think it will ever try...