Refactoring of check category connections

[kvch]

Description

This PR refactors the replication category. Now all checks share the same PostgreSQL connection. The new postgres.LazyConn is not thread-safe. It is used for executing checks sequentially.

Once the check finishes, the cleanup function must be called.

Type of Change

Please select the relevant option(s):

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📚 Documentation update
• 🔧 Refactoring (no functional changes)
• ⚡ Performance improvement
• 🧪 Test coverage improvement
• 🔨 Build/CI changes
• 🧹 Code cleanup

Testing

• Unit tests added/updated
• Integration tests added/updated
• Manual testing performed
• All existing tests pass

Checklist

• Code follows project style guidelines
• Self-review completed
• Code is well-commented
• Documentation updated where necessary

[github-actions]

Merging this branch will decrease overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/xataio/pgstream/cmd	0.00% (ø)
github.com/xataio/pgstream/internal/postgres	28.78% (-0.53%)	👎
github.com/xataio/pgstream/pkg/stream/preflight	25.44% (-1.41%)	👎

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Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/xataio/pgstream/cmd/check_cmd.go	0.00% (ø)	35 (+3)	0	35 (+3)
github.com/xataio/pgstream/internal/postgres/pg_lazy_conn.go	0.00% (ø)	10 (+10)	0	10 (+10)
github.com/xataio/pgstream/pkg/stream/preflight/builder.go	0.00% (ø)	31 (+10)	0	31 (+10)
github.com/xataio/pgstream/pkg/stream/preflight/replication.go	0.00% (ø)	42 (-4)	0	42 (-4)

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.