RoundRobinLoadBalancer enhancement

[hebelala]

RoundRobinLoadBalancer enhancements as follows:

• send yield command as ping, for fail fast
• use ConcurrentHashMap instead of HashMap, to avoid volatile problem with S_OK or S_BAD

[codecov-commenter]

Codecov Report

Merging #357 (1d8a18f) into master (e9935d7) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #357      +/-   ##
============================================
+ Coverage     63.58%   63.59%   +0.01%     
  Complexity      729      729              
============================================
  Files            66       66              
  Lines          3444     3445       +1     
  Branches        501      501              
============================================
+ Hits           2190     2191       +1     
  Misses          915      915              
  Partials        339      339              

Impacted Files	Coverage Δ
...ebula/client/graph/net/RoundRobinLoadBalancer.java	96.07% <100.00%> (+0.07%)	⬆️

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[Nicole00]

Great job