Support offheap cache

[javeme]

implememnt #844
Change-Id: Ib0170b5d6b442231f77dba29e4b2d2270225c935

[codecov]

Codecov Report

Merging #846 into master will increase coverage by 2.92%.
The diff coverage is 79.80%.

@@             Coverage Diff              @@
##             master     #846      +/-   ##
============================================
+ Coverage     69.13%   72.05%   +2.92%     
- Complexity     4503     4674     +171     
============================================
  Files           295      298       +3     
  Lines         22045    22293     +248     
  Branches       3121     3153      +32     
============================================
+ Hits          15240    16063     +823     
+ Misses         5308     4668     -640     
- Partials       1497     1562      +65     

Impacted Files	Coverage Δ	Complexity Δ
...du/hugegraph/backend/cache/CachedBackendStore.java	14.28% <0.00%> (+4.76%)	0.00 <0.00> (ø)
...hugegraph/backend/serializer/BinarySerializer.java	83.24% <0.00%> (-0.06%)	107.00 <0.00> (+3.00)	⬇️
...m/baidu/hugegraph/backend/cache/AbstractCache.java	70.29% <70.29%> (ø)	28.00 <28.00> (?)
...va/com/baidu/hugegraph/backend/cache/RamCache.java	71.83% <71.42%> (-1.89%)	27.00 <9.00> (-24.00)
...om/baidu/hugegraph/backend/cache/OffheapCache.java	76.54% <76.54%> (ø)	16.00 <16.00> (?)
.../com/baidu/hugegraph/backend/cache/LevelCache.java	91.89% <91.89%> (ø)	19.00 <19.00> (?)
...in/java/com/baidu/hugegraph/core/GraphManager.java	55.22% <100.00%> (+4.09%)	0.00 <0.00> (ø)
...om/baidu/hugegraph/backend/cache/CacheManager.java	88.23% <100.00%> (+6.98%)	12.00 <5.00> (+4.00)
...ugegraph/backend/cache/CachedGraphTransaction.java	95.45% <100.00%> (+0.08%)	32.00 <1.00> (ø)
...gegraph/backend/cache/CachedSchemaTransaction.java	96.61% <100.00%> (ø)	28.00 <0.00> (ø)
... and 24 more

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[Linary]

delete it?