Move counterfactual logic for reading price0Cumulative and price1Cumulative to pair

[moodysalem]

When reading price*CumulativeLast variables, all oracle consumers actually want the cumulative price as of the current block, not the last liquidity event (e.g. mint/burn/swap)

We provided a library in uniswap v2 periphery for this
https://github.com/Uniswap/uniswap-v2-periphery/blob/master/contracts/libraries/UniswapV2OracleLibrary.sol#L16

However, we should just add the view method directly to the pair getCumulativePrices() returns (price0Cumulative, price1Cumulative) that returns the cumulative prices as of the current block, even if there was no liquidity event.

Also potentially consider marking the Last suffixed variables private, if we want to force others to use this other method.