Handling recycling race and volatile read-writes in Apache Async client

[eyalkoren]

Fixes the issue reported at https://discuss.elastic.co/t/a-null-pointer-exception-is-possible-when-performing-asynchronous-requests/185315

[codecov-io]

Codecov Report

Merging #662 into master will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             master     #662     +/-   ##
===========================================
  Coverage     61.92%   61.92%             
+ Complexity     1387       68   -1319     
===========================================
  Files           199      199             
  Lines          7743     7743             
  Branches        950      950             
===========================================
  Hits           4795     4795             
  Misses         2651     2651             
  Partials        297      297

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