[pool] Consider marking underpriced UOs as ineligible and not serving to builder

[dancoombs]

Describe the feature
Currently we don't mark underpriced UOs as ineligible. We always serve the highest priced UOs to the builder regardless of their price, and let the builder filter out by price. We could instead mark those UOs as ineligible (similar to how we do it with DA tracking) and remove them from the response.

This seems cleaner, but its unclear if it gets many benefits besides not needing to do the filtering a second time at the builder.