dns local cache

[codecov[bot]]

Codecov Report

Merging #347 into master will decrease coverage by 0.05%. The diff coverage is 70.87%.

@@            Coverage Diff             @@
##           master     #347      +/-   ##
==========================================
- Coverage   74.17%   74.12%   -0.06%     
==========================================
  Files          64       65       +1     
  Lines        5259     5229      -30     
==========================================
- Hits         3901     3876      -25     
+ Misses       1358     1353       -5     

Impacted Files	Coverage Δ
QiniuSDK/Http/Dns/QNDnsCacheFile.m	64.28% <50.00%> (-31.46%)	:arrow_down:
QiniuSDK/Http/Dns/QNInetAddress.m	52.63% <52.63%> (ø)
QiniuSDK/Http/Dns/QNDnsPrefetcher.m	79.12% <95.00%> (+7.15%)	:arrow_up:
QiniuSDK/Http/Dns/QNDnsCacheInfo.m	95.65% <95.65%> (ø)
QiniuSDK/Storage/QNConfiguration.m	100.00% <100.00%> (ø)
QiniuSDK/Storage/QNFormUpload.m	70.32% <0.00%> (-6.60%)	:arrow_down:
QiniuSDK/Common/QNAutoZone.m	91.95% <0.00%> (-2.30%)	:arrow_down:
QiniuSDK/Http/QNHttpResponseInfo.m	81.65% <0.00%> (-0.92%)	:arrow_down:
QiniuSDK/Storage/QNUploadInfoReporter.m	91.76% <0.00%> (-0.42%)	:arrow_down:
... and 5 more

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[forrest-mao]

fix #346