Add PromiseInvalidationToken.cancelWithoutInvalidation()

[lilyball]

It may be useful to have a PromiseInvalidationToken.cancelWithoutInvalidation() method that cancels any associated promises without invalidating the token.

The use-case here is when you want to cancel e.g. a request but you're not kicking off a replacement, so you want any onCancel and always handlers to fire. As the recommended style is to not hold onto Promises directly but to just hold onto the token, we need some way to cancel the associated promises without actually invalidating the token.