added cumulative account borrow and repay stats

[0xmDreamy]

Added stats for cumulativeBorrowAmount, cumulativeRepayAmount and borrowFeesPaid on AccountState. These are intended to be used to calculate the interest paid in total on a cauldron using the following formula:

$$ residualRepayAmount = \begin{cases} \frac{borrowPart \times totalBorrow.elastic}{totalBorrow.base}, & \text{if }totalBorrow.base \neq 0\\ 0, & \text{otherwise} \end{cases} $$

$$ interestPaid = cumulativeRepayAmount + residualRepayAmount - cumulativeBorrowAmount - borrowFeesPaid $$

The number for totalBorrow.elastic should ideally include the unaccrued interest.