Update price feed structure + cascade changes

[ali-behjati]

This PR updates the PriceFeed struct to be like so:

pub struct PriceFeed {
    pub id:    PriceIdentifier,
    price:     Price,
    ema_price: Price,
}

and changes the Price to include publish_time:

pub struct Price {
    pub price:        i64,
    pub conf:         u64,
    pub expo:         i32,
    pub publish_time: UnixTimestamp,
}

All of the utility functions for Price that have multiple prices (like mul) will return the minimum publish time as the resulting publish time.

The rest of the changes are fixing its downstream dependencies, removing old tests for staleness checks, and adding new tests for the new PriceAccount to PriceFeed conversion.

[tompntn]

Nice!

[jayantk]

lgtm. please merge this before addressing the minor comment -- it's a small thing and that will make this easier to review.