Switch all timestamps to nanoseconds

[mstange]

This is just an experiment for now.

I've done a global search & replace to see what would break. The only interesting part of this PR at the moment is the gecko format upgrader. I haven't written a processed format upgrader yet. And the gecko format upgrader isn't complete yet, it needs to handle more marker types.

Some of the potentially more controversial Millisecond -> Nanosecond conversions:

• profile.meta.interval
• GC marker data fields
• Network marker data fields