Refactor lisk-core constants to use const, upper case with underscores

[ManuGowda]

Expected behavior

• The constants should be defined using const
• The constants names should have an upper case with underscores for separators.

Actual behavior

• Lisk-core constants are using vars, this would lead to the mutability of the constants and leading to change in behaviour in the application.
• The constants are defined using lowercase with camelCase for separators.

Steps to reproduce

var constants = require('../helpers/constants.js');
constants.activeDelegates = 300;
Now the activeDelegates count will be changed to 300 from 101.

Which version(s) does this affect? (Environment, OS, etc...)

1.0.0

[4miners]

Reopen, because it was closed automatically by mistake.

[MaciejBaj]

Consecutively in the whole application change var to const when possible, and var to let when it gets mutated.