Store timestamps in nanosecond precision

We currently store timestamps in float format while most interfaces and log formats use some integer representation. Converting these to floating point is a lossy operation.

My proposal is to add an additional `timestamp_ns` argument and attribute to the Message class. On initialization, if the `timestamp` argument is given it would be converted to ns. The `.timestamp` attribute can be dynamically calculated on get and set.

It would be fairly trivial to update the interfaces and log formats to use ns timestamps instead.