Fix data race when updating the raft snapshot and compact threshold

[git-hulk]

This closes #245

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 47.13%. Comparing base (6c56470) to head (e24df29).
Report is 29 commits behind head on unstable.

Files with missing lines	Patch %	Lines
store/engine/raft/node.go	83.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable     #253      +/-   ##
============================================
+ Coverage     43.38%   47.13%   +3.74%     
============================================
  Files            37       43       +6     
  Lines          2971     3906     +935     
============================================
+ Hits           1289     1841     +552     
- Misses         1544     1872     +328     
- Partials        138      193      +55     

Flag	Coverage Δ
unittests	47.13% <83.33%> (+3.74%)	⬆️

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