Use the eviction server to write pages with READGEN_OLDEST set.

Even before the eviction trigger has been reached. This should mean
that we clear those pages out of cache earlier, and hopefully will
save application threads from doing the evictions (at least sometimes).
I don't love this name, but it's was the best of three I came up with. Maybe WT_EVICT_REDUNDANT or WT_EVICT_USELESS