DATAMONGO-843 - Improvements in auditing configuration.

Repositories now declare a fallback MappingContext in case none is configured explicitly to make sure @EnableMongoAuditing also works without an explicit MappingContext bean defined.

AuditingEntityListener is now referring to the IsNewAwareAuditingHandler via an intermediate ObjectFactory to prevent the downstream dependencies from being instantiated eagerly at listener init time.

This is to prevent circular initialization dependencies as Spring accesses ApplicationEventListener beans very early in the container lifecycle to check whether they might be interested in a certain even and just dropped immediately afterwards.

Changed BeanNames.MAPPING_CONTEXT constant to mongoMappingContext to let the XML configuration be consistent with AbstractMongoConfiguration.mongoMappingContext().