Discriminator fields are not mapped; ClassMetadata property change

Discriminators defined at the document level allowed separate name/fieldName options for database/PHP names, respectively. Association discriminators are only defined with a single database name.

For BC, allow document-level mappings to still use "fieldName", but give priority to "name". In ClassMetadata, only a single string (the database name) will be stored, as is already done for association mappings.
Set default discriminator field in ClassMetadataInfo

This adds a constant in ClassMetadataInfo, which means the default field will no longer need to be hard-coded elsewhere in ODM.
Move DocumentManager discriminator resolution method to UnitOfWork

This moves DocumentManager::getClassNameFromDiscriminatorValue() to UnitOfWork::getClassNameForAssociation(). Since the default discriminator field is now set in ClassMetadataInfo, it no longer needs to be hard-coded. DocumentManager::createDBRef() has also been refactored a bit.