Experiment with the page read generation: don't touch the read generation
unless we're actually evicting, increment the read generation in steps so
we (hopefully) only set it every N times we access it.
Be explicit about the fact that setting read-gen to 0 means "not set".
Always return read-generations in the future, the idea is that we can avoid
updating the page generation for a little while after each update, that is,
page generations will proceed in steps, they won't be updated on every access.
We have no proof this works, but it's arguably a good idea and I can't see
any cost.
Merge pull request #468 from wiredtiger/readgen

Don't update page read generation on every read.