This `handler` allows to observe the value of the `input` promise, without changing the propagated value. The `output` promise will be resolved with the same value as the `input` promise (the `handler` returned value will be ignored). However, if `handler` throws, `output` is rejected with the new exception. Unlike `finally`, this handler is not called for rejections.
Make sure that the chained value is not copied when `finally` is called for a fulfilled input promise. The value was copied 7 times in the previous version because it was captured in a lambda, which one copied multiple times.
Make QPromise thread safe but also ensure that continuation lambdas (then/fail/finally) are called in the thread of the promise instance they are attached to.
Make continuation methods const (then/fail/finally) and ensure that the resolved promise value/error is copied only when required, same for user lambdas (dispatching result is now fully handled by the PromiseData).
