improve load backup task executor

[zhoney]

Change-Id: I59e3bf669820614868d465465d99dbc7075391df

[codecov]

Codecov Report

Merging #92 (3e168d8) into gh-pre-release (7b82fbc) will increase coverage by 0.01%.
The diff coverage is 0.00%.

@@                 Coverage Diff                  @@
##             gh-pre-release      #92      +/-   ##
====================================================
+ Coverage             61.96%   61.97%   +0.01%     
+ Complexity             6858     6856       -2     
====================================================
  Files                   430      430              
  Lines                 35797    35800       +3     
  Branches               4947     4948       +1     
====================================================
+ Hits                  22181    22188       +7     
+ Misses                11340    11330      -10     
- Partials               2276     2282       +6     

Impacted Files	Coverage Δ
...om/baidu/hugegraph/task/StandardTaskScheduler.java	75.41% <0.00%> (-0.55%)	⬇️
...egraph/backend/store/cassandra/CassandraStore.java	71.79% <0.00%> (-0.86%)	⬇️
...c/main/java/com/baidu/hugegraph/task/HugeTask.java	71.82% <0.00%> (-0.31%)	⬇️
...du/hugegraph/backend/tx/GraphIndexTransaction.java	84.18% <0.00%> (-0.22%)	⬇️
.../java/com/baidu/hugegraph/auth/RolePermission.java	91.76% <0.00%> (+1.17%)	⬆️
...in/java/com/baidu/hugegraph/auth/HugeResource.java	79.22% <0.00%> (+1.29%)	⬆️
...va/com/baidu/hugegraph/auth/HugeAuthenticator.java	40.21% <0.00%> (+5.43%)	⬆️

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