[#308] Introduce AirConcurrentMap as possible backing HashMap

[pjeli]

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GitHub Issue

Fixes #308

Checklist:

• This pull request updates the documentation
• This pull request changes the code
• Title of the PR is of format (example) : [#25] Add Pull Request Template, where [#25] is the GitHub issue number

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• My commits all reference GH issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
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[codecov]

Codecov Report

Merging #309 (bddc130) into master (2ac9f13) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #309   +/-   ##
=========================================
  Coverage     74.29%   74.30%           
- Complexity     1071     1074    +3     
=========================================
  Files            55       56    +1     
  Lines          7070     7083   +13     
  Branches        638      638           
=========================================
+ Hits           5253     5263   +10     
- Misses         1421     1423    +2     
- Partials        396      397    +1     

Impacted Files	Coverage Δ
...rver/namenode/AirConcurrentMapINodeCollection.java	100.00% <100.00%> (ø)
...doop/hdfs/server/namenode/AbstractQueryEngine.java	66.93% <0.00%> (-0.21%)	⬇️
.../hdfs/server/namenode/analytics/WebServerMain.java	75.52% <0.00%> (-0.18%)	⬇️

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