support property default value of vertex/edge

[javeme]

Change-Id: I654820df78d67886b46dfa733a5309a17c504c58

[codecov]

Codecov Report

Merging #1182 into master will increase coverage by 1.29%.
The diff coverage is 90.00%.

@@             Coverage Diff              @@
##             master    #1182      +/-   ##
============================================
+ Coverage     64.45%   65.75%   +1.29%     
- Complexity     5596     5668      +72     
============================================
  Files           356      356              
  Lines         28929    28949      +20     
  Branches       4066     4069       +3     
============================================
+ Hits          18646    19034     +388     
+ Misses         8395     8019     -376     
- Partials       1888     1896       +8     

Impacted Files	Coverage Δ	Complexity Δ
...main/java/com/baidu/hugegraph/schema/Userdata.java	62.50% <ø> (ø)	5.00 <0.00> (ø)
...n/java/com/baidu/hugegraph/schema/PropertyKey.java	77.69% <33.33%> (-1.05%)	51.00 <1.00> (+1.00)	⬇️
...n/java/com/baidu/hugegraph/structure/HugeEdge.java	73.97% <100.00%> (+0.26%)	83.00 <0.00> (ø)
...ava/com/baidu/hugegraph/structure/HugeElement.java	70.58% <100.00%> (+2.00%)	74.00 <6.00> (+6.00)
...java/com/baidu/hugegraph/structure/HugeVertex.java	77.04% <100.00%> (+0.18%)	83.00 <0.00> (ø)
...du/hugegraph/backend/tx/GraphIndexTransaction.java	81.16% <0.00%> (+0.11%)	201.00% <0.00%> (ø%)
.../baidu/hugegraph/backend/query/ConditionQuery.java	86.40% <0.00%> (+0.40%)	106.00% <0.00%> (+1.00%)
...aidu/hugegraph/backend/store/mysql/MysqlTable.java	81.54% <0.00%> (+0.55%)	96.00% <0.00%> (+1.00%)
.../backend/store/cassandra/CassandraSessionPool.java	57.14% <0.00%> (+1.02%)	11.00% <0.00%> (ø%)
...aidu/hugegraph/backend/tx/AbstractTransaction.java	73.24% <0.00%> (+1.91%)	53.00% <0.00%> (+1.00%)
... and 11 more

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