Naming/MemoizedInstanceVariableName

[johnnyshields]

This PR renames a few internal instance variables to be consistent with their memoizer methods. No one should be accessing these directly.

• lib/mongoid/document.rb - rename @_type to @_types
• lib/mongoid/document.rb - rename @model_cache_key to @model_key
• lib/mongoid/association/relatable.rb - rename @class_name to @relation_class_name
• lib/mongoid/association/relatable.rb - rename @klass to @relation_class
• lib/mongoid/contextual/mongo.rb - make documents_loader a private memoizer method (was an attr_accessor). Can call load_sync publicly to get the loader. this functionality is @api private so no one should be using it.
• lib/mongoid/traversable.rb - Skip @__parent, @__children, and @__descendants as per code comment.