fix clear api create snapshot for all backend(should rocksdb only)

[zhoney]

Change-Id: I936ee13a0a4f8afc61384fe6adee89bbb74c8256

[codecov]

Codecov Report

Merging #1532 (f4a5a66) into master (23beacd) will increase coverage by 14.79%.
The diff coverage is 0.00%.

@@              Coverage Diff              @@
##             master    #1532       +/-   ##
=============================================
+ Coverage     44.77%   59.57%   +14.79%     
- Complexity     4619     6144     +1525     
=============================================
  Files           410      410               
  Lines         33272    33273        +1     
  Branches       4590     4590               
=============================================
+ Hits          14899    19822     +4923     
+ Misses        16633    11379     -5254     
- Partials       1740     2072      +332     

Impacted Files	Coverage Δ
...in/java/com/baidu/hugegraph/StandardHugeGraph.java	68.39% <ø> (+2.23%)	⬆️
...h/backend/store/raft/RaftBackendStoreProvider.java	0.00% <0.00%> (ø)
.../hugegraph/backend/store/rocksdb/RocksDBStore.java	64.04% <0.00%> (-5.17%)	⬇️
...ph/backend/store/AbstractBackendStoreProvider.java	82.41% <0.00%> (-4.40%)	⬇️
...om/baidu/hugegraph/backend/store/BackendStore.java	80.76% <0.00%> (-3.85%)	⬇️
...m/baidu/hugegraph/backend/tx/GraphTransaction.java	79.84% <0.00%> (+0.32%)	⬆️
...ava/com/baidu/hugegraph/structure/HugeElement.java	70.77% <0.00%> (+0.45%)	⬆️
...n/java/com/baidu/hugegraph/structure/HugeEdge.java	76.49% <0.00%> (+0.46%)	⬆️
...du/hugegraph/backend/tx/GraphIndexTransaction.java	81.27% <0.00%> (+0.46%)	⬆️
...om/baidu/hugegraph/task/StandardTaskScheduler.java	75.60% <0.00%> (+0.48%)	⬆️
... and 191 more

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[javeme]

seems can move to RaftStoreProvider.initSystemInfo()