MSK-278 - ProducerRegistryAPI: after initial cache construction producers don't properly unregistered

- equals/hashCode methods are overridden in ProducerReference
- added JUnit test
- during initial cache init one instance of ProducerReference is used in both caches inside ProducerRegistryAPIImpl
Merge pull request #47 from anotheria/MSK-278

MSK-278 - ProducerRegistryAPI: after initial cache construction producer...