Drops zipkin dependency from spring-cloud-sleuth-core

[codefromthecrypt]

There was emmense work to prepare for decoupling of spring-cloud-sleuth-core
from Zipkin. This included complete test conversion and deprecations between
2.2.x and 3.0.x.

This moves all Zipkin related code to spring-cloud-sleuth-zipkin, making the
primary data recording tool SpanHandler as opposed to Reporter<zipkin2.Span>

For example, Wavefront and soon Stackdriver can implement SpanHandler and
skip conversion into the Zipkin model first. SpanHandler also includes
begin and end hooks which allow data extensions to be developed.

see https://github.com/wavefrontHQ/wavefront-spring-boot

[codecov]

Codecov Report

Merging #1649 into master will increase coverage by 0.00%.
The diff coverage is 75.51%.

@@            Coverage Diff            @@
##             master    #1649   +/-   ##
=========================================
  Coverage     59.30%   59.31%           
+ Complexity      768      767    -1     
=========================================
  Files           134      134           
  Lines          4141     4144    +3     
  Branches        487      489    +2     
=========================================
+ Hits           2456     2458    +2     
- Misses         1459     1462    +3     
+ Partials        226      224    -2     

Impacted Files	Coverage Δ	Complexity Δ
...loud/sleuth/autoconfig/TraceAutoConfiguration.java	93.33% <ø> (+14.38%)	15.00 <0.00> (-9.00)	⬆️
.../cloud/sleuth/zipkin2/ZipkinAutoConfiguration.java	78.82% <75.51%> (-4.51%)	17.00 <9.00> (+9.00)	⬇️
...cloud/sleuth/instrument/reactor/ReactorSleuth.java	46.66% <0.00%> (-11.12%)	8.00% <0.00%> (-1.00%)
.../sender/ZipkinRestTemplateSenderConfiguration.java	82.53% <0.00%> (+3.17%)	4.00% <0.00%> (ø%)

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