Replace Agrona with JCTools as it's Java 7 compatible

[felixbarny]

As an additional benefit the Atomic variants of JCTools' queues don't depend on sun.misc.Unsafe, making it Java 11 proof.

[codecov-io]

Codecov Report

Merging #101 into master will not change coverage.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #101   +/-   ##
=========================================
  Coverage     76.95%   76.95%           
  Complexity      853      853           
=========================================
  Files            92       92           
  Lines          2973     2973           
  Branches        246      246           
=========================================
  Hits           2288     2288           
  Misses          546      546           
  Partials        139      139

Impacted Files	Coverage Δ	Complexity Δ
...ain/java/co/elastic/apm/impl/ElasticApmTracer.java	77.48% <100%> (ø)	41 <0> (ø)	⬇️

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