Switch to unix_ts_usec for event t0

[mjkramer]

Paired with DUNE/ndlar_flow#195 which adds the unix_ts_usec field to charge/events. Using it improves the trigger matching between 2x2 and Mx2 in the CAFmaker.

[sindhu-ku]

Is this the same for FSD, and the same for sim and data?
If not maybe we should add an if else based on config?

[mjkramer]

For FSD, and for 2x2 sim, flow just fills unix_ts_usec with the old ts_start field (rescaled from ticks to microseconds). That's configured by the pps_delay_corrector_enabled param of flow's event builder. So no need for extra config or logic in flow2supera!