OGM-207 Making assertions on entity/association count more useable in case of failures

* Also implementing getNumberOfAssociations for entity-embedded entities in MongoDB
OGM-207 Adding API to TestHelper to retrieve association count per storage strategy for document stores
OGM-207 Providing API on test helper to obtain the current store's context type
OGM-207 Moving API for association storage configuration from CouchDB context types to document store context types

* Creating base classes DocumentStoreEntityContextImpl et al.
* Deriving CouchDB context classes from these
* Moving related tests to core