Add a field to HTTP2Config controlling how we behave when an HTTP/2
connection reaches its concurrency limit.
This field will have no effect until golang.org/x/net/http2 is
updated to make use of it, and h2_bundle.go is updated with the
new http2 package.
For #67813
Change-Id: Ic72a0986528abb21649f28e9fe7cf6e1236b388d
Reviewed-on: https://go-review.googlesource.com/c/go/+/615875
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Damien Neil <dneil@google.com> Reviewed-by: Nicholas Husin <nsh@golang.org> Reviewed-by: Nicholas Husin <husin@google.com>