[eth] Gas improvement: Use latestPriceInfoPublishTime when possible

[ali-behjati]

When updating the price we only need to know the publish time of the cached price and don't need to read the whole memory. This change adds a method to get the publish time directly and uses it all the places that is possible within the code base here. We can use it in updatePriceFeedsIfNecessary too, but since it's in the sdk right now we cannot change it. Will do that later when we move the sdk inside this repo.

Snapshot difference:

testBenchmarkGetUpdateFee() (gas: 22 (0.016%)) 
testBenchmarkUpdatePriceFeedsFresh() (gas: -6822 (-1.740%)) 
testBenchmarkUpdatePriceFeedsIfNecessaryFresh() (gas: -7878 (-1.891%)) 
testBenchmarkUpdatePriceFeedsIfNecessaryNotFresh() (gas: -4834 (-2.388%)) 
testBenchmarkUpdatePriceFeedsNotFresh() (gas: -14822 (-4.056%))