Fixes for overflow caching with fast-path eviction.

This refines the changes in 1bb1074495f21f4aab8058aaf80b4d3a5a0c2801 to only apply when safe, which means both that the transactional context is "good enough", and that the page is simple.  Otherwise, we need a real transaction.

refs #743
There's no need to set the page's read generation inside the eviction
lock: I can't imagine it makes a difference, but we know the read
generation is a hot cache line, don't mix-and-match.