LRU walks should not wait for page references to become stable.  This was a bug introduced when switching to using hazard pointers for eviction walks.
Skip files for LRU eviction as long as the queue has a minimum number of entries: it doesn't have to be completely full.
When ascending through a tree in a no-wait walk, we can be blocked (e.g., if a thread has locked an internal page trying to evict it).  Handle that case gracefully in the walk: release the coupled page and end the walk.
whitespace
Clear the "clear walks" flag after servicing the request.