Couple span's start_time and end_time with start and end methods #361
Description
This started as a valid point raised by @toumorokoshi while discussing #358. The SDK Span start_time and end_time can be set by the user manually. This can be a problem because the same attributes are used to determine if the span has started or ended.
Protect these attributes from manual user setting. Raise a warning to the user when an attempt is made to do so.