Declare $proxyFactory property in DocumentManager
Cast slaveOkay to boolean in ClassMetadataInfo setter
Refactor collection and DB getters in DocumentManager
Remove unnecessary use statement
Rewrite DocumentManagerMock to use reflection to expose privates

ClassMetadataFactoryTest and SchemaManagerTest required refactoring for these changes.
Use null default value for ClassMetadataInfo slaveOkay (fixes #759)

This ensures DocumentManager::getDocumentCollection() will not overwrite read preferences by default.
Test ClassMetadataInfo slaveOkay wrt DocumentManager getter (#759)