fix LockManager get/create race condition

[javeme]

Change-Id: I524a2d4d13a99f3943bfa0e4a44aa0ccf9c573a1

[codecov]

Codecov Report

Merging #53 into master will increase coverage by 3.07%.
The diff coverage is 90.00%.

@@             Coverage Diff              @@
##             master      #53      +/-   ##
============================================
+ Coverage     81.76%   84.83%   +3.07%     
- Complexity      686      732      +46     
============================================
  Files            57       57              
  Lines          2089     2097       +8     
  Branches        304      309       +5     
============================================
+ Hits           1708     1779      +71     
+ Misses          251      184      -67     
- Partials        130      134       +4     

Impacted Files	Coverage Δ	Complexity Δ
...va/com/baidu/hugegraph/concurrent/LockManager.java	80.95% <71.42%> (+80.95%)	9.00 <4.00> (+9.00)
...ava/com/baidu/hugegraph/concurrent/AtomicLock.java	74.07% <100.00%> (+54.07%)	11.00 <2.00> (+9.00)
...n/java/com/baidu/hugegraph/concurrent/KeyLock.java	78.43% <100.00%> (+67.06%)	25.00 <18.00> (+22.00)
...n/java/com/baidu/hugegraph/concurrent/RowLock.java	92.85% <100.00%> (+16.77%)	16.00 <15.00> (+6.00)

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