Allow repeatable deletions and enhance thread safety

[figroc]

What's the purpose of this PR

fix two bugs in mockserver

• repeatable deletion of properties is erroneous
• there would be race condition issue on override properties

Which issue(s) this PR fixes:

Fixes #3064

Brief changelog

• change immutable set to concurrent hash set
• change plain hash map to concurrent map

[nobodyiam]

Thanks! Would you please add a test for repeatable deletions so that it would be protected in the future?

[codecov-io]

Codecov Report

Merging #3069 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #3069      +/-   ##
============================================
+ Coverage     51.30%   51.31%   +0.01%     
- Complexity     2234     2235       +1     
============================================
  Files           432      432              
  Lines         13383    13385       +2     
  Branches       1375     1375              
============================================
+ Hits           6866     6869       +3     
  Misses         6042     6042              
+ Partials        475      474       -1     

Impacted Files	Coverage Δ	Complexity Δ
...ip/framework/apollo/mockserver/EmbeddedApollo.java	90.36% <100.00%> (+0.23%)	18.00 <0.00> (ø)
...ervice/service/ReleaseMessageServiceWithCache.java	87.05% <0.00%> (+1.17%)	25.00% <0.00%> (+1.00%)

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[figroc]

@nobodyiam tests added

[nobodyiam]

LGTM