net/http: synchronize "100 Continue" write and Handler writes

The expectContinueReader writes to the connection on the first
Request.Body read. Since a Handler might be doing a read in parallel or
before a write, expectContinueReader needs to synchronize with the
ResponseWriter, and abort if a response already went out.

The tests will land in a separate CL.

Fixes #34902
Fixes CVE-2020-15586

Change-Id: Icdd8dd539f45e8863762bd378194bb4741e875fc
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/793350


Reviewed-by: Filippo Valsorda <valsorda@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/242598


Run-TryBot: Katie Hockman <katie@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>