Adapt RestClient to support connection reuse

[Linary]

Change-Id: I328065488049ec81e507b469104570d8f094f1af

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@33b3724). Click here to learn what that means.
The diff coverage is 41.17%.

@@            Coverage Diff            @@
##             master      #33   +/-   ##
=========================================
  Coverage          ?   73.34%           
  Complexity        ?      472           
=========================================
  Files             ?       89           
  Lines             ?     2015           
  Branches          ?      113           
=========================================
  Hits              ?     1478           
  Misses            ?      450           
  Partials          ?       87

Impacted Files	Coverage Δ	Complexity Δ
...in/java/com/baidu/hugegraph/driver/HugeClient.java	58.82% <38.46%> (ø)	10 <3> (?)
...in/java/com/baidu/hugegraph/client/RestClient.java	80% <50%> (ø)	7 <1> (?)

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