Fixing TRUNK-5452 - Adapt equal signature and implement hashCode

The purpose of the earlier commit was  to  improve CohortMembership's equality check in collections.. For this code to be executed, equals method needs to have an Object in the method signature, not the specific class, like in this case equals(CohortMembership otherCohortMembership)

Steps:

• Adapt equal of CohortMembership to use Object
• Create a hashCode method that is necessary for a Set in Java to work
• Add a test case to validate the functionality 
• Make sure everything else is working just fine

It is based on: https://github.com/openmrs/openmrs-core/commit/60d92bda69d0e46bda832e64bfd6b881320f911f#r31850302