[TODO] used if there is a concurrency issues between gc and flush

[Besroy]

In previous implementation, there may be a concurrency issue between 'permanent_destroy' and 'cp_flush/flush_durable_commit_lsn': if m_rd_sb is destroyed at T1 then is written at T2, a NPE will occur. This pr is a fix way to avoid this issue by adding m_sb_mtx for destroy.

In addition to the current fix, there are two other potential approaches to address this issue:

1. support triggering specific CP: so that HO's pg_destroy won't trigger repl_dev's cp_flush, which acquiring m_rd_map's lock
2. refine leave_group func: modify the leave_group within the nuraft_mesg component to only shut down servers. so that gc can call leave_group and upper destroy without lock, call permanent_destroy and remove it from map with lock

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 42.85714% with 4 lines in your changes missing coverage. Please review.

Project coverage is 66.18%. Comparing base (1a0cef8) to head (e36b56e). Report is 146 commits behind head on master.

Files with missing lines	Patch %	Lines
src/lib/replication/repl_dev/raft_repl_dev.cpp	42.85%	3 Missing and 1 partial :warning:

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #661      +/-   ##
==========================================
+ Coverage   56.51%   66.18%   +9.67%     
==========================================
  Files         108      109       +1     
  Lines       10300    11288     +988     
  Branches     1402     1539     +137     
==========================================
+ Hits         5821     7471    +1650     
+ Misses       3894     3091     -803     
- Partials      585      726     +141     

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[xiaoxichen]

thanks for the effort to refine the edge case.