Add forward compatible start_time, end_time and elapsed_time propertys to result objects

[pekkaklarck]

The plan is to enhance performance of getting and processing timestamps in RF 7.0 (#4258). Part of that is internally representing start and end times as datetime objects and elapsed time as a timedelta. The old starttime, endtime and elapsed time attributes will be preserved for backwards compatibility reasons, and new start_time, end_time and elapsed_time attributes added.

For forward compatibility reasons we should add start_time, end_time and elapsed_time already in RF 6.1. They can be propetys that get their values from the old attributes. In RF 7 we can then change them to be the "real" attributes and make the old ones propertys.