Add MongoDataSource abstraction.

[christophstrobl]

Provide an abstraction over the MongoDB Client / Cluster API as well as the methods to obtain the database factory.

The goal is to have a component that provides access to both database specific API as well as cluster/client related

interface MongoClusterProvider extends MongoSessionProvider {
  MongoCluster getMongoCluster();
}

interface MongoDataSource extends MongoDatabaseFactory {
  MongoClusterProvider clusterProvider();
}

Transaction and session management could move to a TransactionAwareMongoDataSourceProxy delegating to already existing MongoDatabaseUtils but with a more concise approach.