update queue_lens on generation ends.

[lcw99]

When using the SHORTEST_QUEUE method for load balancing, an issue arises because the queue_length isn't updated when a generation task ends. This leads to inaccurate load balancing across worker processes. To address this problem, I implemented two improvements:

1. Added a heartbeat signal that's sent when a generation task completes, ensuring the queue length is updated accurately.
2. Introduced a random selection process for breaking ties when multiple queues have the same length.

These changes should result in more effective and balanced distribution of work across all available worker processes.