refactoring: increase test coverage
Update to CollectionUtils 4.0-alpha1

This is an alpha release, but has been well tested already. The major
changes are the use of generics, varargs and Iterable.

The release notes can be found at:
http://commons.apache.org/proper/commons-collections/release_4_0.html
refactoring: do not duplicate code path, partition list instead
Experiment with concurrent execution