Fix skip leave signal randomly when SnapshotHandle::leave followed by a SnapshotHandle::shutdown immediately.

[al8n]

No description provided.

[codecov]

Codecov Report

Merging #20 (e606e3a) into main (ffaaba2) will increase coverage by 0.46%.
The diff coverage is 90.90%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #20      +/-   ##
==========================================
+ Coverage   67.79%   68.25%   +0.46%     
==========================================
  Files          36       36              
  Lines        4347     4354       +7     
==========================================
+ Hits         2947     2972      +25     
+ Misses       1400     1382      -18     

Files	Coverage Δ
core/src/key_manager.rs	61.15% <100.00%> (+10.74%)	⬆️
core/src/lib.rs	81.81% <100.00%> (+4.04%)	⬆️
core/src/serf/api.rs	61.04% <ø> (+0.37%)	⬆️
core/src/snapshot.rs	73.04% <88.23%> (+0.42%)	⬆️

... and 7 files with indirect coverage changes

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