Change list

Closes #1722:

• Use unified imports order for all classes
• Configure checkstyle rules for imports

Types of changes

What types of changes are you proposing/introducing to Java client?

• No changes in production code.
• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Details

The imports order reflects the default order from IntellJ IDEA (so that developers don't need to configure their IDEs) except for star-imports, they should be avoided.