[#30083][prism] Factor out hold tracking to dedicated structures

[lostluck]

Refactor out hold tracking to dedicated structures, so the logic is not replicated everywhere. Document the purpose of watermark holds within that.

This is yet another change to make the processing time change easier to read. In particular, event time watermark hold management is critical for correct behavior when waiting for processing time triggers.

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[lostluck]

R: @damondouglas

[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[lostluck]

Thanks! PTAL

[damondouglas]

@lostluck