Instrument Runnables/Callables instead of wrapping them

[felixbarny]

What does this PR do?

closes #987

Bonus content:

• Support for ForJoinPool and ScheduledExecutorService
• Support for ExecutorService#invokeAny and ExecutorService#invokeAll

Checklist

• I have updated CHANGELOG.asciidoc
• I have added tests that prove my fix is effective or that my feature works
• [ ] Added an API method or config option? Document in which version this will be introduced
• [ ] I have made corresponding changes to the documentation

[ghost]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request #1206 updated]

• Start Time: 2020-06-17T08:37:23.464+0000

• Duration: 39 min 58 sec

Test stats 🧪

Test	Results
Failed	0
Passed	1391
Skipped	13
Total	1404

[codecov-commenter]

Codecov Report

Merging #1206 into master will increase coverage by 0.37%.
The diff coverage is 44.82%.

@@             Coverage Diff              @@
##             master    #1206      +/-   ##
============================================
+ Coverage     59.38%   59.75%   +0.37%     
- Complexity       86      247     +161     
============================================
  Files           339      343       +4     
  Lines         16847    17305     +458     
  Branches       2468     2560      +92     
============================================
+ Hits          10004    10341     +337     
- Misses         6203     6288      +85     
- Partials        640      676      +36     

Impacted Files	Coverage Δ	Complexity Δ
...java/co/elastic/apm/attach/ElasticApmAttacher.java	18.18% <0.00%> (-0.74%)	3.00 <0.00> (ø)
...va/co/elastic/apm/agent/impl/ElasticApmTracer.java	74.50% <ø> (+4.40%)	43.00 <0.00> (+43.00)
...ava/co/elastic/apm/agent/jmx/JmxMetricTracker.java	65.82% <0.00%> (-1.28%)	0.00 <0.00> (ø)
...in/java/co/elastic/apm/agent/util/GlobalLocks.java	50.00% <50.00%> (ø)	0.00 <0.00> (?)
...c/main/java/co/elastic/apm/agent/util/IOUtils.java	58.09% <80.00%> (+17.89%)	0.00 <0.00> (ø)
...a/co/elastic/apm/agent/report/ApmServerClient.java	83.96% <100.00%> (-5.11%)	0.00 <0.00> (ø)
...lastic/apm/agent/impl/ElasticApmTracerBuilder.java	73.56% <0.00%> (-2.20%)	0.00% <0.00%> (ø%)
...lastic/apm/agent/servlet/AsyncInstrumentation.java	63.04% <0.00%> (-1.41%)	0.00% <0.00%> (ø%)
...astic/apm/agent/impl/transaction/AbstractSpan.java	75.18% <0.00%> (-0.26%)	54.00% <0.00%> (+54.00%)	⬇️
... and 18 more

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