Add `id` to OperationBatch class.
Rename current Operation object to Request, in preparation to redefine "Operation" as a representation for high level methods load(), save(), and destroy().
Add execute() method to Request, which executes the Request using its configured proxy. Also add subclasses for each of the four CRUD request types: CreateRequest, ReadRequest, UpdateRequest, DestroyRequest.
Create new Operation and LoadOperation classes to represent a single high-level load operation, which may consist of one or more underlying requests. Implement in Collection's load() method.
Add ability to cancel load() operations from Collection, using the abort() method on the returned OperationPromise.