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.go

@@ -86,9 +86,11 @@ func NewArchivalTLSOrQUICHandshakeResult(
 		NoTLSVerify:        config.InsecureSkipVerify,
 		PeerCertificates:   TLSPeerCerts(state, err),
 		ServerName:         config.ServerName,
+		T0:                 started.Seconds(),
 		T:                  finished.Seconds(),
 		Tags:               []string{},
 		TLSVersion:         netxlite.TLSVersionString(state.Version),
+		TransactionID:      index,
 	}
 }