fix cross-close the underlying rocksdb session pool

[javeme]

fix: #597
Change-Id: I8b185cd7f81a9a04bc6fd971490ae887fd4ddbb5

[zhoney]

add an empty between args

[zhoney]

this.refCount = origin.refCount.incrementAndGet();

[javeme]

They have to share the address of refCount

[codecov]

Codecov Report

Merging #598 into master will increase coverage by 0.65%.
The diff coverage is 61.11%.

@@             Coverage Diff              @@
##             master     #598      +/-   ##
============================================
+ Coverage     70.46%   71.11%   +0.65%     
- Complexity     3458     3497      +39     
============================================
  Files           216      216              
  Lines         16612    16637      +25     
  Branches       2367     2370       +3     
============================================
+ Hits          11705    11831     +126     
+ Misses         3658     3558     -100     
+ Partials       1249     1248       -1

Impacted Files	Coverage Δ	Complexity Δ
...gegraph/backend/store/rocksdb/RocksDBSessions.java	100% <ø> (ø)	1 <0> (ø)	⬇️
...h/backend/store/rocksdbsst/RocksDBSstSessions.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...raph/backend/store/rocksdbsst/RocksDBSstStore.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...e/src/main/java/com/baidu/hugegraph/HugeGraph.java	73.43% <100%> (+0.56%)	66 <2> (+2)	⬆️
.../hugegraph/backend/store/rocksdb/RocksDBStore.java	75.28% <70%> (+17.2%)	54 <1> (+11)	⬆️
...raph/backend/store/rocksdb/RocksDBStdSessions.java	70.24% <80%> (+0.7%)	29 <2> (+5)	⬆️
...om/baidu/hugegraph/backend/store/BackendTable.java	92% <0%> (-2%)	12% <0%> (ø)
...du/hugegraph/backend/tx/GraphIndexTransaction.java	79.52% <0%> (-0.48%)	146% <0%> (ø)
...va/com/baidu/hugegraph/backend/cache/RamCache.java	72.54% <0%> (+0.39%)	45% <0%> (+1%)	⬆️
... and 13 more

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