Fix csv quoting mechanism to preserve line feeds

[msg555]

Problem

Currently we're using json.dumps to quote/escape each column of our CSV rows. However, json.dumps escapes characters beyond just the double-quote and backslash characters that will not get unescaped when postgres processes the CSV data. In particular the current method will cause the backspace character (\b), form feed (\f), line feed (\n), carriage return (\r), and tab (\t) to be escaped in the extracted payload mistakenly.

Proposed changes

Uses a simpler escaping mechanism that only escapes the double-quote character and backslash literals. This appears to be what postgres is expecting; see https://www.postgresql.org/docs/8.0/sql-copy.html.

We also to take care to encode the empty string as a quoted empty string and null just as an empty string. According to that same sql-copy doc this appears to be what postgres is expecting by default to distinguish between the two cases.

Types of changes

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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
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Checklist

• Description above provides context of the change
• I have added tests that prove my fix is effective or that my feature works
• Unit tests for changes (not needed for documentation changes)
• CI checks pass with my changes
• Bumping version in setup.py is an individual PR and not mixed with feature or bugfix PRs
• Commit message/PR title starts with [AP-NNNN] (if applicable. AP-NNNN = JIRA ID)
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• Commits follow "How to write a good git commit message"
• Relevant documentation is updated including usage instructions