Migrate to go1.12 to simplify our cmd/http package (#7302)

Simplify the cmd/http package overall by removing
custom plain text v/s tls connection detection, by
migrating to go1.12 and choose minimum version
to be go1.12

Also remove all the vendored deps, since they
are not useful anymore.

cmd/http/server.go

@@ -92,11 +92,9 @@ func (srv *Server) Start() (err error) {
 	var listener *httpListener
 	listener, err = newHTTPListener(
 		addrs,
-		tlsConfig,
 		tcpKeepAliveTimeout,
 		readTimeout,
 		writeTimeout,
-		srv.MaxHeaderBytes,
 		srv.UpdateBytesReadFunc,
 		srv.UpdateBytesWrittenFunc,
 	)