Calculate all children span start times using the monotonic clock

[rakyll]

We initially made an improvement to calculate the span duration from the monotonic clock.

We also have to make sure that children start times are calculated with the monotonic clock to have a more precise overview of a trace.

[odeke-em]

After https://github.com/census-instrumentation/opencensus-go/pull/509 is merged, then we can easily work on this too.