fix: Rollback polling of advisory lock in #483

[achimnol]

This PR reverts the busy-polling of advisory locks but preserves the
refactoring of #483 such as introduction of connect_database() utility
function.

In our customer sites, the good-old blocking advisory lock (#482) is proven to
work better, particularly with limited number of maximum database
connections.

Ultimately we should migrate distributed locking to etcd, but it requires
rewriting of the etcd3 client library and thus will be revisited in the
future.

[codecov]

Codecov Report

Merging #503 (bd4441c) into main (3956b1f) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head bd4441c differs from pull request most recent head a9826b2. Consider uploading reports for the commit a9826b2 to get more accurate results

@@            Coverage Diff             @@
##             main     #503      +/-   ##
==========================================
- Coverage   48.84%   48.84%   -0.01%     
==========================================
  Files          54       54              
  Lines        8946     8943       -3     
==========================================
- Hits         4370     4368       -2     
+ Misses       4576     4575       -1     

Impacted Files	Coverage Δ
src/ai/backend/manager/models/utils.py	76.52% <100.00%> (+0.25%)	⬆️

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