Use LRU for entity cache

Closes #3498154

core/assets/scaffold/files/default.services.yml

@@ -238,6 +238,11 @@ parameters:
     # https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Access-Control-Allow-Credentials
     supportsCredentials: false
 
+  # The maximum number of entities stored in memory. Lowering this number can
+  # reduce the amount of memory used in long-running processes like migrations,
+  # however will also increase requests to the database or entity cache backend.
+  entity.memory_cache.slots: 1000
+
   queue.config:
     # The maximum number of seconds to wait if a queue is temporarily suspended.
     # This is not applicable when a queue is suspended but does not specify