Add start / stop timestamps for request metrics

[danielmitterdorfer]

For performance analysis it is useful to correlate timestamps of multiple sources. We capture already quite a few timestamps but it would be great if we could get more details for request metrics:

• For individual requests (i.e. rally-metrics-*) we should get the start and stop timestamp (denoted in milliseconds) as meta-data. It should be possible to override this from the runner, just as we did in #1019.
• For summary metrics (i.e. rally-results-* and rally-races-*) we should determine the minimum start timestamp and the maximum stop timestamp per task and expose it as two new properties. This simplifies correlating timestamps from other diagnostic data with summary metrics from the metrics store.

These timestamps should only be available in the metrics documents but should not be printed in the command line report.