refactor: Improve Postgres init to reuse connection

[cbaker6]

Pull Request

• Report security issues confidentially.
• Any contribution is under this license.
• Link this pull request to an issue.

Issue

Connections are not reused during Postgres init. This is very old leftover TODO from https://github.com/parse-community/parse-server/pull/4375#discussion_r159132356

Closes: n/a

Approach

Reuse connection in transaction.

Tasks

• [ ] Ensure all tests pass

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• ❌ Please link an issue that describes the reason for this pull request, otherwise your pull request will be closed. Make sure to write it as Closes: #123 in the PR description, so I can recognize it.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 83.33333% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 93.78%. Comparing base (c277739) to head (1455ef5). Report is 1 commits behind head on alpha.

Files	Patch %	Lines
...dapters/Storage/Postgres/PostgresStorageAdapter.js	83.33%	4 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##            alpha    #8663      +/-   ##
==========================================
- Coverage   94.14%   93.78%   -0.37%     
==========================================
  Files         186      186              
  Lines       14687    14683       -4     
==========================================
- Hits        13827    13770      -57     
- Misses        860      913      +53     

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