OGM-207

(Based on top of https://github.com/hibernate/hibernate-ogm/pull/271)

This PR makes the association storage strategy configurable for MongoDB and unifies means of configuration that aspect with CouchDB:

* Using `@AssociationStorage` annotation for CouchDB and MongoDB
* Providing `DocumentStore(Global|Entity|Property)Context#associationStorage()`
* Providing new annotation `@AssociationDocumentStorage` and corresponding API for configuring whether association documents should all go into the same collection or a collection per type