Fix race condition

[rmichela]

Fixes #123

[rmichela]

@bsideup Please take a look

[codecov-io]

Codecov Report

Merging #126 into master will decrease coverage by 0.08%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #126      +/-   ##
============================================
- Coverage     36.37%   36.29%   -0.09%     
  Complexity       54       54              
============================================
  Files            27       27              
  Lines           635      631       -4     
  Branches         26       26              
============================================
- Hits            231      229       -2     
+ Misses          392      390       -2     
  Partials         12       12

Impacted Files	Coverage Δ	Complexity Δ
...pc/common/ReactiveStreamObserverPublisherBase.java	64.58% <ø> (-1.42%)	6 <0> (ø)
.../common/ReactiveStreamObserverPublisherClient.java	70.58% <ø> (+3.92%)	6 <0> (ø)	⬇️
.../common/ReactiveStreamObserverPublisherServer.java	40% <ø> (+1.29%)	6 <0> (ø)	⬇️

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