Fix compilation warnings and redundant imports.

Also make sure -Werror is enforced since it was not taking effect
properly due to multiple "compilerArgument" parameters. The correct
way is to use "compilerArgs" which takes a list of "args".
Enforce no warnings in cdk-maven-plugin and cdk-tools.
Add missing @since tags.
Remove dependency on Commons Configuration. This dependency was being
indirectly fulfilled by Hadoop, but it would be better not to introduce
a new explicit dependency in CDK for such a small piece of code.