internal/http3: prevent request body read when 100 status was not sent

Make HTTP/3 pass TestServerExpect100ContinueUnreadBody that was added in
CL 793160.

Also add serverRequestReader so the server will always deal with the
same req.Body. This makes the code simpler and gets rid of the scattered
logic used to coordinate bodyReader and responseWriter.

For golang/go#70914

Change-Id: Ic94c222f39e83e121012ddcd2579a2b66a6a6964
Reviewed-on: https://go-review.googlesource.com/c/net/+/798321
Auto-Submit: Nicholas Husin <nsh@golang.org>
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>