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