I just realized we have a `WT_TREE_CACHE` flag that eviction doesn't set, but then, we set `WT_READ_CACHE_ONLY` in the walk function, if `WT_TREE_EVICT` is set.   Well, the only other thing the `WT_TREE_EVICT` flag does is set `WT_READ_NO_BUMP`, it makes more sense to me to get rid of `WT_TREE_EVICT`, and have eviction set `WT_TREE_CACHE` and a new flag, `WT_TREE_NO_BUMP`.

Admittedly, that means the walk and page-in functions have the same flags, which is a little odd, but renaming the flags to be `WT_READ_XXX` would work, the tree-walk function is "reading" pages.