add batch maximum to volume/deployment watcher

[tgross]

In #7794, @langmartin pointed out that the volume watcher's batcher doesn't have a maximum batch size. This code was lifted from our deployment watcher, so we're missing that there as well. In practice, the batch window is only 250ms so the batch sizes never get large enough with deployment watcher for it to become an issue. But it might be with large jobs for volume watcher, and it's a good safety/robustness check.

[tgross]

Closed by #7907

[github-actions]

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