Let TaskScheduler pause when under loading mode

[Linary]

Change-Id: I593ee46a2a14af18f9105d84249da68592556dde

[codecov]

Codecov Report

Merging #1414 (ecb6d4c) into master (ec974d0) will increase coverage by 0.13%.
The diff coverage is 55.55%.

@@             Coverage Diff              @@
##             master    #1414      +/-   ##
============================================
+ Coverage     62.08%   62.22%   +0.13%     
  Complexity     5854     5854              
============================================
  Files           386      381       -5     
  Lines         32297    32109     -188     
  Branches       4507     4492      -15     
============================================
- Hits          20051    19979      -72     
+ Misses        10209    10092     -117     
- Partials       2037     2038       +1     

Impacted Files	Coverage Δ	Complexity Δ
...java/com/baidu/hugegraph/config/ServerOptions.java	98.36% <ø> (-0.56%)	0.00 <0.00> (ø)
...in/java/com/baidu/hugegraph/StandardHugeGraph.java	68.83% <33.33%> (-0.21%)	119.00 <0.00> (ø)
...ain/java/com/baidu/hugegraph/task/TaskManager.java	70.50% <60.00%> (-2.09%)	27.00 <1.00> (+1.00)	⬇️
...in/java/com/baidu/hugegraph/server/RestServer.java	45.00% <100.00%> (ø)	0.00 <0.00> (ø)
...va/com/baidu/hugegraph/task/ServerInfoManager.java	70.78% <0.00%> (-2.81%)	51.00% <0.00%> (-1.00%)
...in/java/com/baidu/hugegraph/core/GraphManager.java	52.09% <0.00%> (-1.28%)	0.00% <0.00%> (ø%)
...a/com/baidu/hugegraph/backend/query/Condition.java	78.10% <0.00%> (-0.73%)	32.00% <0.00%> (ø%)
...om/baidu/hugegraph/auth/StandardAuthenticator.java	0.00% <0.00%> (ø)	0.00% <0.00%> (ø%)
... and 8 more

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