the current timestamp used in mp protocol and in AImultiplayer is not a good one:
it can pause, or even change speed if we change warp value.
we want it to be used for network protocol lag and jitter estimation, and
a time flowing linearly on both side is needed, here's a first introduction
of this timestamp relates to real elapsed time.
here it's initialised to the system clock, then follow the monotonic clock.
in future improvement, it will allow time synchronisation betwen mp players,
to have a very good close formation flight experience.
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