Release 3.8.0

• Don't error if job hasn't been submitted yet (resolves #1666)
• Match doc to reality for --preserve-environment
• CWL: symlink jobStore files instead of copying
• When killing tasks, make sure to reap child processes in the process groups
• Better error checking in piped docker commands
• Fix case where a job has been put in currentjobs, but not yet dispatc…
• Use hard links instead of copied files on import to file job store
• CWL: Add support for InitalWorkDirRequirement
• Set proper value for service job preemptable requirement
• Use chunked transfer in AWS job store importing
• Lower timeout for domain binding in Toil Clean
• Add import os.