Set default-constructed timestamp to current time in StartSpan

[isaachier]

Fixes #14

[rnburn]

If one of the system/steady timestamps is specified but not the other, shouldn't this try to convert the provided timestamp instead of using now?

Also, I think this same sort of logic is missing for finish as well.

[isaachier]

Good point. Let me add both of those suggestions.

[codecov]

Codecov Report

Merging #18 into master will increase coverage by 0.04%.
The diff coverage is 89.47%.

@@            Coverage Diff             @@
##           master      #18      +/-   ##
==========================================
+ Coverage   85.44%   85.49%   +0.04%     
==========================================
  Files          93       93              
  Lines        2219     2240      +21     
  Branches      202      205       +3     
==========================================
+ Hits         1896     1915      +19     
- Misses        252      254       +2     
  Partials       71       71

Impacted Files	Coverage Δ
src/jaegertracing/Span.cpp	54.23% <100%> (+2.45%)	⬆️
src/jaegertracing/Tracer.cpp	91.11% <86.66%> (-0.89%)	⬇️
src/jaegertracing/Span.h	92.23% <0%> (+0.23%)	⬆️

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