fix cache metrics perf issue

[javeme]

test perf on Mac (8 CPUs) with threads=10 loop=8kw:

	ConcurrentHashMap	RamCache	RamCache metrics
1	3.8~6.5s	5.5~9.1s	none
2	5.4	7.1	volatile enabledMetrics=false
3	5.4	7.8	volatile enabledMetrics=true
4	5.1	28.9	volatile hits
5	5.1	9.4	LongAdder hits

[imbajin]

and here are some fire svgs for perf case with kout: (save as .svg check the details)

• use volatile to count
  
• use LongAdder to count
  
• no count (use original ConcurrentHashMap)
  

[codecov]

Codecov Report

Merging #1515 (0dfc5e9) into master (0c51de5) will decrease coverage by 0.54%.
The diff coverage is 88.57%.

@@             Coverage Diff              @@
##             master    #1515      +/-   ##
============================================
- Coverage     60.02%   59.47%   -0.55%     
- Complexity      305     6114    +5809     
============================================
  Files           426      406      -20     
  Lines         35096    33066    -2030     
  Branches       4970     4574     -396     
============================================
- Hits          21065    19666    -1399     
+ Misses        11894    11336     -558     
+ Partials       2137     2064      -73     

Impacted Files	Coverage Δ
...om/baidu/hugegraph/backend/cache/CacheManager.java	87.03% <66.66%> (-1.20%)	⬇️
...va/com/baidu/hugegraph/backend/cache/RamCache.java	72.47% <83.33%> (+0.31%)	⬆️
...m/baidu/hugegraph/backend/cache/AbstractCache.java	77.00% <91.66%> (+5.84%)	⬆️
...ugegraph/backend/cache/CachedGraphTransaction.java	84.33% <100.00%> (+0.09%)	⬆️
...om/baidu/hugegraph/backend/cache/OffheapCache.java	57.83% <100.00%> (ø)
...raph/backend/store/postgresql/PostgresqlStore.java	0.00% <0.00%> (-100.00%)	⬇️
...raph/backend/store/postgresql/PostgresqlTable.java	0.00% <0.00%> (-96.83%)	⬇️
...kend/store/postgresql/PostgresqlStoreProvider.java	0.00% <0.00%> (-92.50%)	⬇️
...aph/backend/store/postgresql/PostgresqlTables.java	0.00% <0.00%> (-87.96%)	⬇️
...raph/backend/store/cassandra/CassandraMetrics.java	0.00% <0.00%> (-86.96%)	⬇️
... and 60 more

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