InMemoryBagUserStateFactory creates a cache token per state cell

When the state cache is enabled in the Python SDK, the batch mode of the Flink Runner currently only allows a single user state cell because a new cache token is generated for each state cell; the caching code in the Python SDK Harness only supports one cache token per user state handler.

Theoretically multiple cache tokens would work but would just be adding to the payload. We should make sure to just send a single cache token in batch mode (which is already the case in streaming)