Just trying to reduce some boilerplate. I kept this as a separate commit to
illustrate clearly how this type of things is done since we'll have people
learning TypeScript.
* src/types/misc.ts (NodeCallback<T,R>): New type.
* src/server/dapi/TokenedDataApi.ts: Use it.
* test/server/dapi/TokenedDataApiTest.ts: Use it.
d8c065817f