http2: Fix issue where slow requests are terminated prematurely

[hyperair]

This PR fixes an issue where http2 requests taking longer than 500ms will be terminated prematurely, with http2.request silently resolving with truncated responses instead of rejecting with an error.

The issue comes from starting the idle shutdown timer (500ms) immediately after the beginning of the latest request on the session, and not detecting that the session is still actively transferring data before forcefully terminating the session.

This PR fixes that by switching the connection idle shutdown timer to session.setTimeout instead of setTimeout.

I've also verified that this fix makes the client in https://github.com/hyperair/google-http2-bug-demo work correctly (more info here):

• all requests return the correct untruncated data
• client process shuts down properly after all requests are done

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Fixes #768 🦕

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