x/net/http2: make Server reject conn-specific request headers

[bradfitz]

The Servers side of #15085, etc.

If the http2.Server gets a request with Connection or Keep-Alive etc headers (see https://http2.github.io/http2-spec/#rfc.section.8.1.2.2), then it should reject it.

[bradfitz]

Dup of #14214