Sleep even when file already existed.

[DeadSix27]

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) (kind of)
• Question
• Other

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• I've verified and I assure that I'm running youtube-dl 2017.11.26
• At least skimmed through the README, most notably the FAQ and BUGS sections
• Searched the bugtracker for similar issues including closed ones

It could be similar to: #4924 but afaik that post had no fix, this one does.

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In the common downloader: https://github.com/rg3/youtube-dl/blob/master/youtube_dl/downloader/common.py#L351
The code does not sleep when a file is already downloaded, despite sleep_interval being set which is an issue on some sites, e.g NicoNico, which even when you do not download the file give you a error 503 when requesting too many videos in short time.

One way to solve this would be something like:
https://github.com/DeadSix27/youtube-dl/commit/4a386648cf85511d9eb283ba488858b6a5dc2444.patch but there are probably other approaches too.