caches: make a set copy to edit it during operation

rhodecode/lib/rc_cache/cache_key_meta.py

@@ -38,7 +38,7 b' def free_cache_keys(*args):'
 
     if cache_keys_by_pid:
         try:
-            for cache_proc in cache_keys_by_pid:
+            for cache_proc in set(cache_keys_by_pid):
                 like_expression = '{}%'.format(cache_proc)
                 CacheKey.query().filter(
                     CacheKey.cache_key.like(like_expression)).delete(synchronize_session='fetch')