Fix macos free_memory calculate by using vm_stat

[coderzc]

fixed #1315

[codecov]

Codecov Report

Merging #1396 (2c01161) into master (3d73961) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1396   +/-   ##
=========================================
  Coverage     62.05%   62.05%           
- Complexity     5837     5838    +1     
=========================================
  Files           386      386           
  Lines         32185    32185           
  Branches       4494     4494           
=========================================
  Hits          19974    19974           
  Misses        10178    10178           
  Partials       2033     2033           

Impacted Files	Coverage Δ	Complexity Δ
...a/com/baidu/hugegraph/backend/query/Condition.java	78.10% <0.00%> (-0.37%)	32.00% <0.00%> (ø%)
...c/main/java/com/baidu/hugegraph/task/HugeTask.java	69.84% <0.00%> (+0.30%)	82.00% <0.00%> (+1.00%)

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