The eviction server walks the cache when populating the queue, we attempt to make it so that the walk never waits for a page. There are cases when the server needs to wait for a page to transition to a different state (in WT_REF_READING or WT_REF_LOCKED). We should add statistics so we can see if the eviction server ends up waiting for a surprisingly long time.
This would give us a way to notice if
WT-4565 is happening.