[I] [C++] Refine reset of Span to be reusable [arrow]

[via GitHub]

ZENOTME opened a new issue, #47040:
URL: https://github.com/apache/arrow/issues/47040

   ### Describe the enhancement requested
   
   Original reset will cause Span can't be used again, e.g.
   
   ```
   Span span;
   span.reset();
   span.valid(); // crash
   RewrapSpan(span, ..); // crash
   ```
   
   Instead of reset the pointer to SpanImpl, maybe we should reset content 
inside SpanImpl.
   
   ### Component(s)
   
   C++


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