class HangingGetHelper

A helper class for managing [hanging get

semantics](https://fuchsia.dev/fuchsia-src/development/api/fidl.md#delay-responses-using-hanging-gets).

It responds with the most recently updated value.

For each hanging get method in a FIDL interface, like GetData() -> ( Data response ), create one

of these classes. Any time the response should change, call Update(Data x). Any time the client

calls GetFoo(), set the callback on this helper. Once the callback has been set and the data has

been updated, the callback will be triggered with the new data.

Each callback will only be triggered once. Each Update will only trigger, at most, a single

callback. Update(Data x) is idempotent. Calling it with the same value will not trigger a new

execution of a registered callback, nor will it remove the registered callback.