ParallelForEach: using variables

## Description
When running steps in parallel, it is not possible to transfer data between activities of step.
If use variables, the data is overwritten for all steps.

## Steps to Reproduce

1. Create a parallel loop.
2. For each step, perform an operation that returns a result.
3. Put this result in a variable.

``` csharp
public class TestWorkflow : WorkflowBase
{
protected override void Build(IWorkflowBuilder builder)
{
Variable stepVariable = builder.WithVariable().WithWorkflowStorage();

var sendHttpRequest = new SendHttpRequest
{
Url = new Input(new Uri("https://www.google.com/search?q=111111111")),
ParsedContent = new Output(stepVariable)
};

var stepWriteLine = new WriteLine(stepVariable);

var parallel = new ParallelForEach
{
Items = new Input(new Expression("JavaScript", "[\"first\", \"second\"]")),
Body = new Flowchart
{
Activities =
{
sendHttpRequest,
stepWriteLine
},
Connections =
{
new Connection(sendHttpRequest, stepWriteLine)
}
}
};

var initWriteLine = new WriteLine("Line");

builder.Root = new Flowchart
{
Start = initWriteLine,
Activities =
{
initWriteLine,
parallel
},
Connections =
{
new Connection(initWriteLine, parallel)
}
};
}
}
```