Replacible distributed lock backend

[achimnol]

• fix: A bug in PgAdvisoryLock
• ci: Upgrade mypy to the latest version that supports Python 3.10
• feat: Implement replacible distributed lock backend
• feat: Set the default lock backend to "etcd"

[codecov]

Codecov Report

Merging #571 (d98be7b) into main (4d74bd7) will decrease coverage by 0.11%.
The diff coverage is 39.58%.

@@            Coverage Diff             @@
##             main     #571      +/-   ##
==========================================
- Coverage   49.17%   49.05%   -0.12%     
==========================================
  Files          55       54       -1     
  Lines        9109     9130      +21     
==========================================
  Hits         4479     4479              
- Misses       4630     4651      +21     

Impacted Files	Coverage Δ
src/ai/backend/manager/config.py	63.73% <ø> (ø)
src/ai/backend/manager/server.py	57.77% <18.51%> (-2.59%)	⬇️
src/ai/backend/manager/api/context.py	68.29% <50.00%> (-0.94%)	⬇️
src/ai/backend/manager/scheduler/dispatcher.py	27.09% <50.00%> (+0.18%)	⬆️
src/ai/backend/manager/types.py	83.33% <57.14%> (-10.79%)	⬇️
src/ai/backend/manager/idle.py	30.52% <66.66%> (-0.11%)	⬇️
src/ai/backend/manager/cli/context.py	69.81% <100.00%> (-0.56%)	⬇️
src/ai/backend/manager/defs.py	100.00% <100.00%> (ø)
src/ai/backend/manager/models/utils.py	76.59% <100.00%> (ø)
... and 1 more

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