net/http: use TCP Keep-Alives on DefaultTransport's connections

Update #3362

Also set a 30 second timeout, instead of relying on the
operating system's timeout, which if often but not always 3
minutes.

LGTM=crawshaw
R=rsc, crawshaw
CC=golang-codereviews
https://golang.org/cl/68330046

src/pkg/net/http/transport.go

@@ -30,7 +30,13 @@ import (
 // and caches them for reuse by subsequent calls. It uses HTTP proxies
 // as directed by the $HTTP_PROXY and $NO_PROXY (or $http_proxy and
 // $no_proxy) environment variables.
-var DefaultTransport RoundTripper = &Transport{Proxy: ProxyFromEnvironment}
+var DefaultTransport RoundTripper = &Transport{
+	Proxy: ProxyFromEnvironment,
+	Dial: (&net.Dialer{
+		Timeout:   30 * time.Second,
+		KeepAlive: 30 * time.Second,
+	}).Dial,
+}
 
 // DefaultMaxIdleConnsPerHost is the default value of Transport's
 // MaxIdleConnsPerHost.