Make `Span` start/end times be high-resolution timestamps

[draffensperger]

Currently the start and end times in the Span interface use the Date type, which is accurate only to milliseconds.

But it would be nice to have higher timestamp accuracy as sub-millisecond times can be useful to measure fast events or small differences between client and service spans for an RPC. My suggestion would be to migrate to use the Timestamp interface or a [seconds, nanos] tuple.

We now have the getTimestampWithProcessHRTime, which would be used to populate the times.

This would be a breaking change, but I think doing it sooner rather than later would reduce the pain of it.

[draffensperger]

We may be able to make this less of a breaking change by introducing new field names for Span for the high resolution start/end times but keep start and end as property accessors that return Date. Then eventually those old fields could be deprecated and removed. Now sure how much we are concerned about backwards compatibility at this point but that is an option.