timer: support join inflight task

[ehds]

fix issue #5

support join inflight task.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 75.00000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 77.42%. Comparing base (69d60c3) to head (0a890f4).
Report is 1 commits behind head on main.

Files	Patch %	Lines
src/braft/repeated_timer_task.cpp	62.50%	3 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main       #6      +/-   ##
==========================================
- Coverage   77.89%   77.42%   -0.47%     
==========================================
  Files          63       63              
  Lines        9732     9747      +15     
  Branches     1345     1355      +10     
==========================================
- Hits         7581     7547      -34     
- Misses       2150     2198      +48     
- Partials        1        2       +1     

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