Reduce database pressure

[tylerpayne]

Leaving in draft while I work with Wenyue to test.

Reduce two high-pressure operations:

1. Recording empty FetchMessages actions
2. Returning all messages for an agent every time.

To solve 1, the protocol can return a boolean flag indicating whether an executed action should be persisted.

To solve 2, track how many messages each agent fetched last, and use that to indicate "empty" or not.

To test, run these three in order

magentic-marketplace run data/mexican_3_9
magentic-marketplace run data/mexican_3_9 --drop-empty-fetch-messages 
magentic-marketplace run data/mexican_3_9 --drop-all-fetch-messages

Then run

magentic-marketplace list --limit 3

And you should see the first has the least actions, then the second, then third has the most.

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PR Checklist (do not remove):

• I've added necessary new tests and they pass
• My PR description explains how to test this contribution
• I have linked this PR to an issue
• I have requested reviews from two people

[willeppy]

My vote would be to just merge the change around not writing fetches since seems less disruptive (under a flag). Default behavior is same as now and then you can set flag to not write fetches

[Wenyueh]

experiments on it work perfectly

[tylerpayne]

@willeppy I know we talked about removing the NOT IN expression and only keeping the drop-empty-fetch logic. But without the NOT IN logic a fetch is never empty 😖

So I've added logic to the SimpleMarketplaceProtocol to track how many messages each agent fetched last, and using that to indicate if a FetchMessages action is "empty" or not. To reduce impact on our message passing logic, I still opted to send all messages to the agents and let the agents filter them out as they were before.

[willeppy]

@tylerpayne non-urgent but think would still be good to merge this soon to make dbs smaller, esp since we've already tagged an arxiv release