fix: Temporarily rollback the default distributed lock impl.

[achimnol]

In our internal testing, etcetra-based distributed lock seems to malfunction when spurious lock timeouts.
Until we find and fix the root cause, temporarily fallback to the prior implementation.
(This is why I've made it as a configurable option!)

[codecov]

Codecov Report

Merging #592 (ccef690) into main (91d7ca8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #592   +/-   ##
=======================================
  Coverage   48.43%   48.43%           
=======================================
  Files          53       53           
  Lines        9131     9131           
=======================================
  Hits         4423     4423           
  Misses       4708     4708           

Impacted Files	Coverage Δ
src/ai/backend/manager/config.py	63.73% <ø> (ø)

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