Don't retry eviction if last_eviction_timestamp is not set

There is a bug in __wt_page_evict_retry, where not having last_eviction_timestamp set is considered to have never evicted the page. On a standalone node without oplog, since there are no timestamps, we would get in a pathological case of retrying eviction repeatedly for some pages. This seems to be the cause of the stall seen in SERVER-48395. The check in last_eviction_timestamp needs to be enhanced to consider the timestamp only when actually set to a value other than none.