Hmm, this isn't straightforward.

The main part of the answer is that the eviction server owns `btree->evict_page`, so we can't change that while the eviction server might be walking a file.  The other places we call `__wt_evict_clear_tree_walk` are either in the eviction server (the call in `bt_walk.c`), or after we have switched the tree into a "no eviction" state.