mixing of stdout/stderr with --parallel

## Expected Behavior
- the stdout of different steps ran in parallel when using `--parallel` shouldn't be mixxed
- it would be nice if `cwltool` had an option to output a log file

## Actual Behavior
When running a workflow with the `--parallel` flag, stdout/stderr from steps running in parallel get mixed together. Would it be possible to prevent this in some way? Maybe by waiting until the step is complete before logging? Another solution could be prepending the job name to each line of the output (so each line would start with `[job workflow_name] `) when a flag is used, like `--timestamps`. This could be useful even when `--parallel` isn't used.

It would also be useful if `cwltool` had a flag to output logging information to a file as well as stdout/stderr.

## Workflow Code
I don't have a simple workflow showing thishat I can share. I can create one if that is useful.

## Your Environment
* cwltool version: 1.0.20180912090223