[release-branch.go1.13-security] 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>
(cherry picked from commit b5e504f4a07c572744b228fa1b10e3989c4c44f3)
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/793499