Add parallel downloading of audio and video

[cheater]

Make sure you are using the latest version: run youtube-dl --version and ensure your version is 2018.02.08. If it's not, read this FAQ entry and update. Issues with outdated version will be rejected.

• I've verified and I assure that I'm running youtube-dl 2018.02.08

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• At least skimmed through the README, most notably the FAQ and BUGS sections
• Searched the bugtracker for similar issues including closed ones
• Checked that provided video/audio/playlist URLs (if any) are alive and playable in a browser

What is the purpose of your issue?

• Bug report (encountered problems with youtube-dl)
• Site support request (request for adding support for a new site)
• Feature request (request for a new functionality)
• Question
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Description of your issue, suggested solution and other information

I would like youtube-dl to download audio and video in parallel, if it decides to download both separately. Currently I have to wait for the video to download, be done, then for the audio to download, and only then does the muxing begin. The download would be much faster if the audio were downloaded while the video was being downloaded.

[cheater]

@dstftw what is this a duplicate of? I have searched the bug tracker before posting and have found nothing.

[cheater]

Just to clarify: this is not about "having parallel downloads". It's about downloading a single video, where youtube-dl ends up downloading two files, one for audio, and one for video. I would like this to happen in parallel.

[dstftw]

#6319.