Worker selection

AFAIK, this is not possible right now: it would be great to be able to specify a particular worker for a particular future. For instance, something like

```r
f <- future(some_computation, worker=worker_id)
```

Consider, e.g., an extension of the use case presented in #300, in which we are exploiting an online API and we want to set up `n` workers to handle `n` distinct API methods and their corresponding rate limitations.