Dynamic forking: inconsistent behavior

### Environment

Highland: 3.0.0.beta-5
Browser: Chrome 61

### Example
```es6
const numbers = Stream();
const even = numbers.fork().filter(x => x % 2 === 0);
const odd = numbers.fork().filter(x => x % 2 !== 0);

even.each((x) => { console.log("even", x); });
odd.fork().each((x) => { console.log("odd", x); });

numbers.write(1);
numbers.write(2);

console.log("> five is added");
const five = odd.fork().filter(x => x === 5);

console.log("> late write");
numbers.write(4);
numbers.write(3);

console.log("> late write 2");
numbers.write(6);
numbers.write(5);

console.log("> consume five");
five.each((x) => { console.log("five", x); })
```

The output:
```
odd 1
even 2
> five is added
> late write
even 4
> late write 2
> consume five
odd 3
even 6
odd 5
five 5
```

### Problem
During "late write" step `even` path is getting processed, while on "late write 2" step it is paused.
This is inconsistent as it should be either paused from the beginning or getting processed all the time (which in my opinion is an option that grants a broader set of capabilities).

Reproduce the reported example with Highland 3.0.0.beta-5 and compare the two late-write stages. Then trace the dynamic fork and stream-consumption entry points to determine why the path pauses inconsistently, and add a regression test that captures the agreed behavior. The issue does not name source files or an existing test.

Written by the indexing model from the issue text.