internal/http3: implement HTTP/3 clientConn methods

Implement Err, Reserve, Release, Available, and InFlight methods in our
HTTP/3 implementation. This allows clientConn to implement the
genericClientConn interface in net/http.

Given that we have no good understanding yet of when one might want to
create multiple HTTP/3 connections to the same server, Available will
always return MaxInt.

For golang/go#70914

Change-Id: Ib1316b33195b6f5d584c5a371d60cddf6a6a6964
Reviewed-on: https://go-review.googlesource.com/c/net/+/784500
Reviewed-by: Nicholas Husin <husin@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>