Make log correlation GA

[felixbarny]

• Load the MDC with the application class loader, as opposed to the
  Thread's context classloader (fixes Stop using Thread context class loader for log correlation #720)
• Remove span.id MDC entry
  This causes the most allocations as in contrast to transaction.id and
  trace.id, the String manifestation can't be reused.
  Other agents also don't add the span.id so it makes sense to align on that
• Docs improvements
• Mention slf4j in supported technologies page

closes #724

• Implement code
• Add tests
• Update documentation
• Update CHANGELOG.md
• Update supported-technologies.asciidoc

[felixbarny]

run the tests

[eyalkoren]

Comments and what we discussed regarding avoiding hard reference to the CL.
In addition, positive test for the fallback context CL.
And lastly- is there common agreement between agents whether the span ID is logged or not?

[codecov-io]

Codecov Report

Merging #742 into master will increase coverage by 0.09%.
The diff coverage is 72.93%.

@@ Coverage Diff @@ ## master #742 +/- ## ============================================ + Coverage 64.79% 64.89% +0.09%  + Complexity 169 68 -101  ============================================ Files 209 210 +1 Lines 8925 8955 +30 Branches 1147 1157 +10 ============================================ + Hits 5783 5811 +28  + Misses 2788 2784 -4  - Partials 354 360 +6

Impacted Files	Coverage Δ	Complexity Δ
...apm/agent/bci/bytebuddy/CustomElementMatchers.java	70.27% <ø> (-3.3%)	0 <0> (ø)
...lastic/apm/agent/report/ReporterConfiguration.java	100% <ø> (ø)	0 <0> (ø)	⬇️
...stic/apm/agent/jdbc/ConnectionInstrumentation.java	81.81% <0%> (-2.8%)	0 <0> (ø)
.../elastic/apm/agent/jdbc/helper/JdbcHelperImpl.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...tic/apm/agent/configuration/CoreConfiguration.java	97.72% <100%> (+0.77%)	0 <0> (ø)	⬇️
...astic/apm/agent/impl/transaction/AbstractSpan.java	83.92% <100%> (+0.29%)	0 <0> (ø)	⬇️
...a/co/elastic/apm/agent/jdbc/helper/JdbcHelper.java	100% <100%> (ø)	0 <0> (?)
...a/co/elastic/apm/agent/report/ReporterFactory.java	100% <100%> (ø)	0 <0> (ø)	⬇️
...astic/apm/agent/report/ApmServerHealthChecker.java	96.55% <100%> (+31.33%)	0 <0> (ø)	⬇️
...lastic/apm/agent/impl/ElasticApmTracerBuilder.java	77.94% <100%> (+1.37%)	0 <0> (ø)	⬇️
... and 14 more

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