Waku sync timestamps compatibility

Waku sync is very sensitive to timestamps.

https://github.com/waku-org/waku-simulator/blob/5f0dfb50eaf0be4040ab1fd5327a399560c0b541/tools/rest-traffic/traffic.py#L19

Python ways of formatting timestamps does not mesh well with nwaku. The number of decimals must be correct.

Unix time in seconds is 10 decimals BUT nwaku uses nanoseconds and so 9 decimals must be added.