[Solo repl dev] Fix log dev flush timer cancel race

[yamingk]

Only impact Solo Repl Dev which enables timer based flush thread:

• Fixed a race (happens ocasionally) that cancel_timer can finish later than the main fiber which will cause asan issue.

Testing:

Run 100 times for the test case:

for i in {1..100};  do ./Debug/src/tests/test_meta_blk --gtest_filter=VMetaBlkMgrTest.write_to_full_test --use_file=true ; done

Cross tested wit HomeBlocks as well to run 100 times.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 67.23%. Comparing base (1a0cef8) to head (a088645).
Report is 197 commits behind head on master.

Files with missing lines	Patch %	Lines
src/lib/logstore/log_dev.cpp	91.66%	0 Missing and 1 partial ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##           master     #723       +/-   ##
===========================================
+ Coverage   56.51%   67.23%   +10.71%     
===========================================
  Files         108      109        +1     
  Lines       10300    11984     +1684     
  Branches     1402     1666      +264     
===========================================
+ Hits         5821     8057     +2236     
+ Misses       3894     3093      -801     
- Partials      585      834      +249     

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[sanebay]

nit: You can use folly::Unit too.