Move Utils#next_id and constants to Tracing::Utils

[delner]

What does this PR do?

Moves tracing-specific Core::Utils#next_id and Tracing::Span ID constants to Tracing::Utils.

Motivation

Eliminate co-dependency for span/trace ID generation, but moving ID generation behavior to the tracing library. This also eliminates dependency on loading & referencing a Span for ID generation (which is generally unnecessary.)

[codecov-commenter]

Codecov Report

Merging #2463 (88af816) into master (c213f61) will increase coverage by 0.00%.
The diff coverage is 99.07%.

@@           Coverage Diff           @@
##           master    #2463   +/-   ##
=======================================
  Coverage   98.02%   98.02%           
=======================================
  Files        1108     1110    +2     
  Lines       59756    59770   +14     
=======================================
+ Hits        58576    58591   +15     
+ Misses       1180     1179    -1     

Impacted Files	Coverage Δ
lib/datadog/core/utils.rb	100.00% <ø> (+2.27%)	⬆️
spec/datadog/core/utils_spec.rb	98.27% <ø> (-0.36%)	⬇️
lib/datadog/tracing/utils.rb	93.75% <93.75%> (ø)
lib/datadog/opentracer/distributed_headers.rb	100.00% <100.00%> (ø)
lib/datadog/tracing/distributed/helpers.rb	100.00% <100.00%> (ø)
lib/datadog/tracing/sampling/rate_sampler.rb	100.00% <100.00%> (ø)
lib/datadog/tracing/span.rb	97.56% <100.00%> (-0.09%)	⬇️
lib/datadog/tracing/span_operation.rb	81.21% <100.00%> (+0.09%)	⬆️
lib/datadog/tracing/trace_operation.rb	98.88% <100.00%> (+<0.01%)	⬆️
...pec/datadog/opentracer/distributed_headers_spec.rb	100.00% <100.00%> (ø)
... and 12 more

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