feat(measurexlite): add T0 and TransactionID (#879)

The T0 field is the moment when we started collecting data, while T
is the moment when we finished collecting data.

The TransactionID field will be repurposed for step-by-step measurements
to indicate related observations collected as part of the same flow
(e.g., TCP+TLS+HTTP).

Note that, for now, this change will only affect measurexlite and we're
not planning on changing other libraries for measuring.

Part of https://github.com/ooni/probe/issues/2137

internal/measurexlite/tls_test.go

@@ -167,6 +167,7 @@ func TestNewTLSHandshakerStdlib(t *testing.T) {
 					NumBytes:  0,
 					Operation: "tls_handshake_done",
 					Proto:     "",
+					T0:        time.Second.Seconds(),
 					T:         time.Second.Seconds(),
 					Tags:      []string{},
 				}
@@ -329,6 +330,7 @@ func TestNewTLSHandshakerStdlib(t *testing.T) {
 					NumBytes:  0,
 					Operation: "tls_handshake_done",
 					Proto:     "",
+					T0:        time.Second.Seconds(),
 					T:         time.Second.Seconds(),
 					Tags:      []string{},
 				}