clear cache when graph close

[javeme]

fix https://github.com/hugegraph/hugegraph/issues/1075#issuecomment-653015753
Change-Id: I774798069c2c5ba25838b3e38eeddc6b126a6e9d

[codecov]

Codecov Report

Merging #1078 into master will increase coverage by 8.50%.
The diff coverage is 77.77%.

@@             Coverage Diff              @@
##             master    #1078      +/-   ##
============================================
+ Coverage     60.97%   69.48%   +8.50%     
- Complexity     4718     5567     +849     
============================================
  Files           331      331              
  Lines         26877    26880       +3     
  Branches       3842     3842              
============================================
+ Hits          16389    18677    +2288     
+ Misses         8719     6389    -2330     
- Partials       1769     1814      +45     

Impacted Files	Coverage Δ	Complexity Δ
...n/java/com/baidu/hugegraph/structure/HugeEdge.java	74.74% <0.00%> (+0.13%)	85.00 <2.00> (+4.00)
...java/com/baidu/hugegraph/structure/HugeVertex.java	77.55% <0.00%> (+1.66%)	85.00 <2.00> (+13.00)
...gegraph/backend/cache/CachedSchemaTransaction.java	96.74% <100.00%> (+11.50%)	32.00 <1.00> (+4.00)
.../java/com/baidu/hugegraph/task/HugeServerInfo.java	68.70% <0.00%> (ø)	18.00% <0.00%> (ø%)
...du/hugegraph/backend/tx/GraphIndexTransaction.java	80.50% <0.00%> (+0.35%)	193.00% <0.00%> (+3.00%)
...u/hugegraph/backend/store/mysql/MysqlSessions.java	68.94% <0.00%> (+0.45%)	27.00% <0.00%> (ø%)
.../baidu/hugegraph/backend/tx/SchemaTransaction.java	91.54% <0.00%> (+0.49%)	70.00% <0.00%> (+1.00%)
...ava/com/baidu/hugegraph/structure/HugeElement.java	70.52% <0.00%> (+0.52%)	70.00% <0.00%> (+3.00%)
...m/baidu/hugegraph/backend/tx/GraphTransaction.java	80.95% <0.00%> (+0.55%)	302.00% <0.00%> (+14.00%)
... and 125 more

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