Fix ConcurrentDictionary (#129)

[StefH]

• Fix ConcurrentDictionary (Random test failures between WireMock.Net 1.0.3.1 and 1.0.3.2 #129)
• Change code to get a free port

[codecov]

Codecov Report

Merging #144 into master will increase coverage by <.01%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master     #144      +/-   ##
==========================================
+ Coverage   67.79%   67.79%   +<.01%     
==========================================
  Files          80       80              
  Lines        2931     2928       -3     
  Branches      394      393       -1     
==========================================
- Hits         1987     1985       -2     
  Misses        727      727              
+ Partials      217      216       -1

Impacted Files	Coverage Δ
src/WireMock.Net/Owin/WireMockMiddleware.cs	67.79% <100%> (ø)	⬆️
src/WireMock.Net/Owin/WireMockMiddlewareOptions.cs	100% <100%> (ø)	⬆️
src/WireMock.Net/Http/PortUtil.cs	88.88% <100%> (+3.17%)	⬆️
src/WireMock.Net/ResponseMessage.cs	85.71% <100%> (ø)	⬆️
src/WireMock.Net/Server/FluentMockServer.Admin.cs	40.03% <50%> (ø)	⬆️
src/WireMock.Net/Server/FluentMockServer.cs	50.41% <50%> (ø)	⬆️

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