Switch to our fork of atomix-storage

CONTROLLER-2069 imports atomix-{storage,utils} source code which has been abandoned by our upstream.

Switch akka-segmented-journal to use this code instead of pulling in the last version published by atomix.io.

This entails a number of steps:

1. add a LICENSE file which makes it clear this code is licensed under Apache License 2.0
2. plug the current code into build, while ignoring checkstyle/spotbugs (for now) and ensuring the right license is packaged (i.e. override EPL packaging)
3. eliminate atomix-utils by rehosting the few classes used by atomix-storage into that component
4. update the segmented-journal pom.xml to use internal atomix-storage
5. eliminate unused atomix-storage classes, we really want just the segmented journal part
6. move Kryo shading hack to atomix-storage artifact

The end result should be that:

1. classes are still in io.atomix namespace
2. artifacts are in org.opendaylight.controller namespace
3. there is no impact on serialization format of akka-segmented-journal