Add additional tests for replace member

[sanebay]

Add OneMemberRestart , LeaderReplace, OneMemberDown test case.
Comments added in start of unit test

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Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 63.71%. Comparing base (1a0cef8) to head (2ee5446).
Report is 83 commits behind head on master.

Files with missing lines	Patch %	Lines
...rc/lib/replication/repl_dev/raft_state_machine.cpp	0.00%	4 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #574      +/-   ##
==========================================
+ Coverage   56.51%   63.71%   +7.20%     
==========================================
  Files         108      109       +1     
  Lines       10300    10696     +396     
  Branches     1402     1468      +66     
==========================================
+ Hits         5821     6815     +994     
+ Misses       3894     3201     -693     
- Partials      585      680      +95     

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[JacksonYao287]

generally, LGTM!

[xiaoxichen]

i still dont think skipping wait_for_commit() on other leader is right....but as it is an UT, we can revisit if we see it flips.

[JacksonYao287]

LGTM！