WiredTiger treats dead trees the same as other trees in eviction

WiredTiger evicts from dead trees the same as other trees, that is, it doesn't preferentially select pages from dead trees (the read-generation is used and so a page from a dead tree might be seen as more valuable than a page from a live tree), nor does it focus on dead trees, it takes the same number of pages from a dead tree to consider for eviction as it does from a live tree.