Provide Support for HTTP/2.

URL to the HTTP/2 RFC

Implementation should only include a supportable TLS ALPN implementation.

This includes the scenario where Undertow is used as a proxy (e.g. in combination with mod-cluster) and HTTP2 is used for communication from client to server.

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Note: options discussed on thecore mailing list:
http://post-office.corp.redhat.com/archives/thecore/2016-June/msg00311.html
http://post-office.corp.redhat.com/archives/thecore/2016-June/msg00296.html