DATAREST-227: Break up AbstractWebIntegrationTest into simpler parts

Decouple the test machinery of this class from the barrage of common test cases used against the various data stores. This way, other test suites that use the same integration approach don't have to be a part of this class hierarchy. 

Also move utilities and assertion methods into separate utility classes to slim down the class hierarchy.
Please add some JavaDoc here. When shall this class be extended by subtypes instead of `AbstractWebIntegrationTests`?