Replace deprecated net2 crate with socket2

[JohnTitor]

cc actix/actix-web#1516

[codecov]

Codecov Report

Merging #140 into master will increase coverage by 0.05%.
The diff coverage is 68.75%.

@@            Coverage Diff             @@
##           master     #140      +/-   ##
==========================================
+ Coverage   61.14%   61.19%   +0.05%     
==========================================
  Files          80       80              
  Lines        5034     5036       +2     
==========================================
+ Hits         3078     3082       +4     
+ Misses       1956     1954       -2     

Impacted Files	Coverage Δ
actix-testing/src/lib.rs	51.06% <0.00%> (ø)
actix-server/src/builder.rs	53.30% <87.50%> (+0.41%)	⬆️
actix-server/tests/test_server.rs	100.00% <100.00%> (ø)
actix-connect/tests/test_connect.rs	87.69% <0.00%> (+3.07%)	⬆️

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