fix: use publisher latest slot for staleness based pricing

[ali-behjati]

This PR will use the latest slot of the publisher for staleness pricing. The issues that this solve is a publisher publishing for a not yet live feed and also a publisher publishing during market close hours. We can roll out this change to all the publishers without the market-close change and it should be similarly effective.

This change also includes changing the default variables for staleness gap and max paid price for priority fees.

• staleness gap rationale: the rpc might be a bit behind by a few slots. Considering this delay and the exponential pricing this should change the unit price when the feed is really getting stale and doubling if it is still stale.
• max paid price: the lowered value will result in almost 5m priority fee which is very high.

We will modify them later based on the performance of the publishers.

[Reisen]

Can you update the PR text with a little bit more clarity around the numbers, what you expect the burn-rate to be practically in the scenario this covers?

[ali-behjati]

I added some comments on the code.