refactor(measurexlite): make buffered channels private (#864)

Closes https://github.com/ooni/probe/issues/2215

internal/engine/experiment/simplequicping/simplequicping.go

@@ -182,7 +182,7 @@ func (m *Measurer) quicHandshake(ctx context.Context, index int64,
 	}
 	_, err := dialer.DialContext(ctx, "udp", address, tlsConfig, &quic.Config{})
 	ol.Stop(err)
-	sp.QUICHandshake = <-trace.QUICHandshake
+	sp.QUICHandshake = trace.FirstQUICHandshakeOrNil() // record the first handshake from the buffer
 	sp.NetworkEvents = append(sp.NetworkEvents, trace.NetworkEvents()...)
 	return sp
 }

internal/engine/experiment/tcpping/tcpping.go

@@ -141,7 +141,7 @@ func (m *Measurer) tcpConnect(ctx context.Context, index int64,
 	ol.Stop(err)
 	measurexlite.MaybeClose(conn)
 	sp := &SinglePing{
-		TCPConnect: <-trace.TCPConnect,
+		TCPConnect: trace.FirstTCPConnectOrNil(), // record the first connect from the buffer
 	}
 	return sp
 }

internal/engine/experiment/tlsping/tlsping.go

@@ -176,7 +176,7 @@ func (m *Measurer) tlsConnectAndHandshake(ctx context.Context, index int64,
 	sni := m.config.sni(address)
 	ol := measurexlite.NewOperationLogger(logger, "TLSPing #%d %s %s %v", index, address, sni, alpn)
 	conn, err := dialer.DialContext(ctx, "tcp", address)
-	sp.TCPConnect = <-trace.TCPConnect
+	sp.TCPConnect = trace.FirstTCPConnectOrNil() // record the first connect from the buffer
 	if err != nil {
 		ol.Stop(err)
 		return sp
@@ -191,7 +191,7 @@ func (m *Measurer) tlsConnectAndHandshake(ctx context.Context, index int64,
 	}
 	_, _, err = thx.Handshake(ctx, conn, config)
 	ol.Stop(err)
-	sp.TLSHandshake = <-trace.TLSHandshake
+	sp.TLSHandshake = trace.FirstTLSHandshakeOrNil() // record the first handshake from the buffer
 	sp.NetworkEvents = trace.NetworkEvents()
 	return sp
 }