improve rocksdb count()

[javeme]

Change-Id: Ia33227952c44f65de2f9e2669a14aa6a9a501cfb
also improve #952

[codecov]

Codecov Report

Merging #988 into master will increase coverage by 0.52%.
The diff coverage is 67.56%.

@@             Coverage Diff              @@
##             master     #988      +/-   ##
============================================
+ Coverage     70.47%   70.99%   +0.52%     
- Complexity     4976     4999      +23     
============================================
  Files           311      311              
  Lines         23942    23961      +19     
  Branches       3349     3350       +1     
============================================
+ Hits          16872    17012     +140     
+ Misses         5420     5297     -123     
- Partials       1650     1652       +2     

Impacted Files	Coverage Δ	Complexity Δ
...u/hugegraph/backend/store/hbase/HbaseSessions.java	61.70% <0.00%> (-0.40%)	23.00 <0.00> (ø)
...aidu/hugegraph/backend/store/hbase/HbaseStore.java	70.56% <0.00%> (-0.93%)	39.00 <0.00> (ø)
...gegraph/backend/store/rocksdb/RocksDBSessions.java	100.00% <ø> (ø)	1.00 <0.00> (ø)
.../hugegraph/backend/store/BackendEntryIterator.java	62.29% <50.00%> (ø)	17.00 <2.00> (ø)
...du/hugegraph/backend/store/BackendSessionPool.java	73.61% <66.66%> (+0.37%)	16.00 <1.00> (ø)
.../hugegraph/backend/store/rocksdb/RocksDBTable.java	72.38% <75.00%> (-3.59%)	35.00 <1.00> (-1.00)
...raph/backend/store/rocksdb/RocksDBStdSessions.java	69.72% <87.50%> (+0.49%)	30.00 <0.00> (ø)
...om/baidu/hugegraph/backend/store/BackendEntry.java	74.28% <100.00%> (+0.95%)	1.00 <0.00> (ø)
... and 8 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 26c4498...a8c4e98. Read the comment docs.

[javeme]

before: cost 3min

"task_name": "g.E().count()",
"task_progress": 0,
"task_create": 1588865631049,
"task_status": "success",
"task_update": 1588865811528,
"task_result": "[263234988]",

after: cost 1.82min (60% time)

"task_name": "g.E().count()",
"task_progress": 0,
"task_create": 1588867770149,
"task_status": "success",
"task_update": 1588867879439,
"task_result": "[258501410]"