replace http client implementation with interface

[vdiskg]

make it easier to customize http client

[nobodyiam]

I think we may still keep the class HttpUtil and delegate its implementation to the HttpClient because some application might already reference it. I did a quick search in github, and found the possibility is true.

To avoid future misuse, we could mark it as @Deprecated.

[codecov-io]

Codecov Report

Merging #3594 (7420982) into master (f192c77) will decrease coverage by 0.21%.
The diff coverage is 77.61%.

@@             Coverage Diff              @@
##             master    #3594      +/-   ##
============================================
- Coverage     51.00%   50.78%   -0.22%     
  Complexity     2328     2328              
============================================
  Files           448      449       +1     
  Lines         14006    14066      +60     
  Branches       1424     1434      +10     
============================================
  Hits           7144     7144              
- Misses         6373     6433      +60     
  Partials        489      489              

Impacted Files	Coverage Δ	Complexity Δ
...com/ctrip/framework/apollo/util/http/HttpUtil.java	0.00% <ø> (-75.00%)	0.00 <0.00> (-9.00)
.../framework/apollo/util/http/DefaultHttpClient.java	75.00% <75.00%> (ø)	9.00 <9.00> (?)
...amework/apollo/internals/ConfigServiceLocator.java	89.58% <100.00%> (ø)	20.00 <0.00> (ø)
...ip/framework/apollo/internals/DefaultInjector.java	68.00% <100.00%> (ø)	3.00 <0.00> (ø)
.../apollo/internals/RemoteConfigLongPollService.java	77.71% <100.00%> (-0.61%)	28.00 <0.00> (-1.00)
...ework/apollo/internals/RemoteConfigRepository.java	89.37% <100.00%> (ø)	25.00 <0.00> (ø)
...ervice/service/ReleaseMessageServiceWithCache.java	85.88% <0.00%> (+1.17%)	24.00% <0.00%> (+1.00%)

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