Round arithmetic is using standard Math functions

[karmacoma]

For reliable number accuracy and consistency with the rest of application, all math operations such as floor, ceil, addition, subtraction should be conducted using the bignum.js: https://github.com/LiskHQ/lisk/blob/development/helpers/bignum.js.

Affected modules:

• https://github.com/LiskHQ/lisk/blob/development/logic/round.js
• https://github.com/LiskHQ/lisk/blob/development/modules/rounds.js

[karmacoma]

@MaciejBaj: Please start with unit tests against existing round logic. Adding failing context for potential number accuracy problem. Then make code changes. Thanks.

[karmacoma]

@MaciejBaj: Once we've proven through unit tests, it might require rebuilding of memory tables. This we also need check.