Ensure is_recording is false and all methods are no-ops after span ends

[morigs]

This PR is targeting to resolve #265. It moves span processing logic from SpanInner's drop to a separate method and calling it on end. This method takes ownership of inner.data.span_data so with_data always returns None.

is_recording checks inner.data and inner.data.span_data now and returns false if some of this options is None. Because of two nested options this check looks kind of ugly 😞

[codecov]

Codecov Report

Merging #341 into master will increase coverage by 0.61%.
The diff coverage is 96.42%.

@@            Coverage Diff             @@
##           master     #341      +/-   ##
==========================================
+ Coverage   53.40%   54.02%   +0.61%     
==========================================
  Files          71       71              
  Lines        5861     5879      +18     
==========================================
+ Hits         3130     3176      +46     
+ Misses       2731     2703      -28     

Impacted Files	Coverage Δ
opentelemetry/src/sdk/trace/span.rs	84.75% <96.42%> (+11.64%)	⬆️

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[jtescher]

Coming along well, few small comments

[jtescher]

Looks good, thanks @morigs