Use integers for timestamps

[julienw]

We're using floats as timestamps. This works well enough for big numbers, but this has a lot of imprecisions. This produces the following issues:

• rounding errors for the marker chart, where the stop of 1 marker is very close to the start of the next marker, they can end up to different lines even though they should belong to the same line. The result is that tree-shaped tracing-like data are badly drawn.
• using the call tree-like data structure for tracing data is more difficult than it could

We should move to using integers everywhere. IIRC using the nanosecond unit should work for us, but we should double check that we don't exceed MAX_SAFE_INTEGER. Maybe working in units of 10ns or 100ns would be as good and would be safer.

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