Fix stop_serving in proactor loop kill all servers

[julien-duponchelle]

Current implementation of the asyncio Proactor event loop has an issue, when you stop a server it's cancel the futures of all other servers.

[the-knights-who-say-ni]

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[julien-duponchelle]

Hi,

http://bugs.python.org/ account updated and CLA signed.

Thanks

[julien-duponchelle]

The cla is now OK on https://bugs.python.org/

[julien-duponchelle]

Hi,

The CLA not signed status seem not updated on Github but it's ok on bugs.python.org.

Do you need something else from me?