Pause background state machine when purging existing logs

[Besroy]

When baseline re-sync is triggered, the background commit thread may execute simultaneously. If the baseline re-sync purges all logs, the commit thread cannot fetch any logs to commit. This change pauses the commit during log purging to prevent the commit thread from accessing non-existent logs.

More details in issue37

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.

Project coverage is 66.08%. Comparing base (1a0cef8) to head (ec55634).
Report is 154 commits behind head on master.

Files with missing lines	Patch %	Lines
src/lib/replication/repl_dev/raft_repl_dev.h	0.00%	5 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #663      +/-   ##
==========================================
+ Coverage   56.51%   66.08%   +9.57%     
==========================================
  Files         108      109       +1     
  Lines       10300    11338    +1038     
  Branches     1402     1542     +140     
==========================================
+ Hits         5821     7493    +1672     
+ Misses       3894     3115     -779     
- Partials      585      730     +145     

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