Add overflow and underflow checks for all operations

[loredanacirstea]

Make sure we have asserts for all smart contracts operations (e.g. +, -, *)

E.g. for settleChannel , getSettleTransferAmounts

        total_deposit_available = (
            participant1_deposit +
            participant2_deposit -
            participant1_locked_amount -
            participant2_locked_amount
        );

        // Overflow and underflow checks
        require(total_deposit_available <= (participant1_deposit + participant2_deposit));
        require(total_deposit_available > 0);

        participant1_amount = (
            participant1_deposit +
            participant2_transferred_amount -
            participant1_transferred_amount
        );

        // Overflow and underflow checks
        require(participant1_amount <= participant1_deposit + participant2_transferred_amount);
        require(participant1_amount > ?);