Bug fix for official-stockfish#2030

With the present code a finished run with run["cores"]>0 will
never be evicted from the cache. This is not a legal
condition but it happened in the past because of race conditions.

This explains why official-stockfish#2030 seemed to create more unvalidated runs.
The illegal runs become more and more likely to be selected for
validation as they are not evicted and eventually dominate the
cache.

server/fishtest/rundb.py

@@ -486,10 +486,11 @@ def flush_buffers(self):
                         if self.scavenge(run):
                             with self.run_cache_write_lock:
                                 self.runs.replace_one({"_id": run["_id"]}, run)
-                    if (
-                        run["cores"] <= 0
-                        and cache_entry["last_access_time"] < now - 300
-                    ):
+                    # Presently run["finished"] implies run["cores"]==0 but
+                    # this was not always true in the past.
+                    if (run["cores"] <= 0 or run["finished"]) and cache_entry[
+                        "last_access_time"
+                    ] < now - 300:
                         del self.run_cache[r_id]
                 elif cache_entry["last_sync_time"] < old:
                     old = cache_entry["last_sync_time"]