refact: let kout degree applied to all labels & remove source vertex from kneighbor default results

[imbajin]

Contains 3 point:

1. kout degree applied to all labels
2. remove source vertex from default kneighbor result
3. fix kneighbor result with limit may loss data because the duplicate elements of current layer are not excluded

PS: Also remove LimitIterator (moved to common) & add/adapt test for them on client#122

[codecov]

Codecov Report

Merging #1459 (1a174db) into master (1ede97c) will increase coverage by 0.36%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1459      +/-   ##
============================================
+ Coverage     61.80%   62.17%   +0.36%     
- Complexity     5840     5863      +23     
============================================
  Files           381      381              
  Lines         32148    32177      +29     
  Branches       4504     4504              
============================================
+ Hits          19869    20005     +136     
+ Misses        10231    10119     -112     
- Partials       2048     2053       +5     

Impacted Files	Coverage Δ	Complexity Δ
...m/baidu/hugegraph/backend/tx/GraphTransaction.java	80.02% <ø> (+0.06%)	306.00 <0.00> (ø)
.../hugegraph/backend/store/rocksdb/RocksDBStore.java	69.61% <0.00%> (-1.79%)	71.00% <0.00%> (+1.00%)	⬇️
.../com/baidu/hugegraph/auth/StandardAuthManager.java	93.20% <0.00%> (-0.99%)	62.00% <0.00%> (ø%)
...c/main/java/com/baidu/hugegraph/task/HugeTask.java	69.53% <0.00%> (-0.31%)	81.00% <0.00%> (-1.00%)
...ugegraph/backend/store/raft/StoreSnapshotFile.java	0.00% <0.00%> (ø)	0.00% <0.00%> (ø%)
...ugegraph/backend/store/raft/StoreStateMachine.java	0.00% <0.00%> (ø)	0.00% <0.00%> (ø%)
...va/com/baidu/hugegraph/backend/page/PageState.java	97.29% <0.00%> (+0.07%)	14.00% <0.00%> (ø%)
...du/hugegraph/backend/tx/GraphIndexTransaction.java	81.15% <0.00%> (+0.11%)	203.00% <0.00%> (ø%)
...a/com/baidu/hugegraph/backend/query/Condition.java	78.46% <0.00%> (+0.36%)	32.00% <0.00%> (ø%)
.../baidu/hugegraph/backend/query/ConditionQuery.java	86.40% <0.00%> (+0.40%)	106.00% <0.00%> (+1.00%)
... and 7 more

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