Base: Improve readability of code

Improve readability of org.adempiere.base code by:

1. Update Javadoc for variables and methods.

2. Add in code comments.

3. Remove commented code.

4. Use static constant for string constant.

5. Mark deprecated variables and methods.

6. Where it help to improve readability, rename private instance variables and method names.

To ease review, will break this into multiple pull request.