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/conn.go

@@ -134,13 +134,15 @@ func addrStringIfNotNil(addr net.Addr) (out string) {
 func NewArchivalNetworkEvent(index int64, started time.Duration, operation string, network string,
 	address string, count int, err error, finished time.Duration) *model.ArchivalNetworkEvent {
 	return &model.ArchivalNetworkEvent{
-		Address:   address,
-		Failure:   tracex.NewFailure(err),
-		NumBytes:  int64(count),
-		Operation: operation,
-		Proto:     network,
-		T:         finished.Seconds(),
-		Tags:      []string{},
+		Address:       address,
+		Failure:       tracex.NewFailure(err),
+		NumBytes:      int64(count),
+		Operation:     operation,
+		Proto:         network,
+		T0:            started.Seconds(),
+		T:             finished.Seconds(),
+		TransactionID: index,
+		Tags:          []string{},
 	}
 }