Reusable Streams

Introduce capability of reusing Streams.

```rust
let mut app = Application::with_conf(conf);
let source_stream = app.iterator((0..1000000).map(|x| Event { id: x, data: 1.5 }), |conf| {
  conf.set_timestamp_extractor(|x: &Event| x.id);
});  
let s1 = source_stream.map(|x| x+ 10);
let s2 = source_stream.flatmap(|x| 0..x);
```