Upstream HTTP/2 support

[motor]

The following post is based on the assumptions that 1. upstream HTTP/2
support is not available in nginx at the moment, and 2. said feature is not
firmly excluded from nginx's future road map :)

We've been internally using an nginx patch (authored by me) which provides
this feature in our nginx instances that front-end etcd servers.
Please let me know if this patch can be a candidate for review and potential
merge.

In case the answer is yes, a heads up: said implementation has been subject
to a few 'compromises' due to internal deadlines, so I might need a fair bit
of technical guidance/discussion to iron out some design/implementation
issues.

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,275391,275391#msg-275391