fix: Add lock lifetime hints to ensure liveness

[achimnol]

refs lablup/backend.ai-common#136

[codecov]

Codecov Report

Merging #575 (01401ad) into main (db46372) will decrease coverage by 0.05%.
The diff coverage is 25.00%.

@@            Coverage Diff             @@
##             main     #575      +/-   ##
==========================================
- Coverage   48.69%   48.64%   -0.06%     
==========================================
  Files          53       53              
  Lines        9101     9101              
==========================================
- Hits         4432     4427       -5     
- Misses       4669     4674       +5     

Impacted Files	Coverage Δ
src/ai/backend/manager/idle.py	30.52% <ø> (ø)
src/ai/backend/manager/scheduler/dispatcher.py	27.09% <ø> (ø)
src/ai/backend/manager/server.py	57.53% <0.00%> (-1.73%)	⬇️
src/ai/backend/manager/types.py	83.33% <100.00%> (ø)
src/ai/backend/manager/models/utils.py	74.46% <0.00%> (+1.41%)	⬆️

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