Cherry-pick: Fix reading from read-only PG source databases (#1238)

[GingerAdonis]

Upstream PR

dimitri#1238

Summary

Recent postmodern versions changed default transaction from BEGIN to BEGIN TRANSACTION ISOLATION LEVEL READ COMMITTED READ WRITE, breaking read-only PostgreSQL source databases. Fix adds a with-pgsql-transaction-read macro using READ COMMITTED READ ONLY isolation level.

Files

• src/package.lisp
• src/pgsql/connection.lisp
• src/sources/pgsql/pgsql.lisp

Notes

• Touches 3 files, +26/-2 lines
• Good fix but needs testing with our postmodern version
• Priority: Lower — evaluate carefully