Fix solo repl dev log flush and graceful shutdown.

[sanebay]

Add flush mode to logdev as nublocks uses timer,
nuobject uses explicit log flush mode. Flush mode has to be stored in superblk to support recovery. Enable solo repl dev UT. Add graceful shutdown for UT to work.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 68.75000% with 15 lines in your changes missing coverage. Please review.

Project coverage is 67.17%. Comparing base (1a0cef8) to head (11bf9b9).
Report is 167 commits behind head on master.

Files with missing lines	Patch %	Lines
src/lib/replication/repl_dev/solo_repl_dev.cpp	44.44%	9 Missing and 1 partial ⚠️
src/lib/replication/service/generic_repl_svc.cpp	84.61%	1 Missing and 1 partial ⚠️
src/lib/logstore/log_dev.cpp	80.00%	0 Missing and 1 partial ⚠️
.../lib/replication/log_store/home_raft_log_store.cpp	66.66%	1 Missing ⚠️
src/lib/replication/repl_dev/solo_repl_dev.h	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##           master     #681       +/-   ##
===========================================
+ Coverage   56.51%   67.17%   +10.66%     
===========================================
  Files         108      109        +1     
  Lines       10300    11428     +1128     
  Branches     1402     1566      +164     
===========================================
+ Hits         5821     7677     +1856     
+ Misses       3894     3001      -893     
- Partials      585      750      +165     

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[yamingk]

LGTM, except the comment around open_logdev to see if we can even simplify, thanks!
Let me know if it makes sense.

[xiaoxichen]

I am curious why the flush_mode needs to be persisted?

The flush mode controlled the flushing behavior when appending logs, once the log dev closed then re-opened, the previous flush_mode should not take any forward impact right? Meaning that nublox can use explicit then in one tick we change the config file to use timer, after a restart the timer can works and effect on new appending logs.

What is the benefit of remember "this log dev was created with explicit flushing" ?

And if we believe there is a racing between logdev_superblk_found and open_logdev, then there is a ambiguity whether the mode provided by open_logdev take effect, or the mode in sb take effect.

[sanebay]

I am curious why the flush_mode needs to be persisted?

The flush mode controlled the flushing behavior when appending logs, once the log dev closed then re-opened, the previous flush_mode should not take any forward impact right? Meaning that nublox can use explicit then in one tick we change the config file to use timer, after a restart the timer can works and effect on new appending logs.

What is the benefit of remember "this log dev was created with explicit flushing" ?

And if we believe there is a racing between logdev_superblk_found and open_logdev, then there is a ambiguity whether the mode provided by open_logdev take effect, or the mode in sb take effect.

This is to support logdev_superblk_found to open logdev with flush mode.