support paging for tasks API

[javeme]

Change-Id: Ie036d433ef74d7c6724bf869425c3fd528f08941
related: #718

[javeme]

Do not upgrade the api version because all client components need to be updated, and it's compatible to add a query param.

[codecov]

Codecov Report

Merging #720 into master will decrease coverage by 0.01%.
The diff coverage is 55%.

@@             Coverage Diff              @@
##             master     #720      +/-   ##
============================================
- Coverage     71.41%   71.39%   -0.02%     
+ Complexity     4328     4327       -1     
============================================
  Files           280      280              
  Lines         20593    20609      +16     
  Branches       2893     2902       +9     
============================================
+ Hits          14706    14714       +8     
- Misses         4444     4446       +2     
- Partials       1443     1449       +6

Impacted Files	Coverage Δ	Complexity Δ
...ava/com/baidu/hugegraph/backend/page/PageInfo.java	86.36% <ø> (+1.91%)	13 <0> (-1)	⬇️
...aidu/hugegraph/backend/page/PageEntryIterator.java	83.72% <100%> (ø)	17 <0> (ø)	⬇️
...n/java/com/baidu/hugegraph/task/TaskScheduler.java	70.38% <53.33%> (-1.11%)	44 <6> (+1)
...main/java/com/baidu/hugegraph/api/job/TaskAPI.java	43.47% <55.55%> (-3.34%)	0 <0> (ø)
...va/com/baidu/hugegraph/backend/cache/RamCache.java	73.33% <0%> (-0.4%)	50% <0%> (-1%)
...a/com/baidu/hugegraph/backend/query/Condition.java	78% <0%> (+0.41%)	31% <0%> (ø)	⬇️

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