Move SQL escape functions to the Postgres package

[cbandy]

Checklist:

• Have you added an explanation of what your changes do and why you'd like them to be included?
• Have you updated or added documentation for the change, as applicable?
• Have you tested your changes on all related environments with successful results, as applicable?
  • Have you added automated tests?

Type of Changes:

• Testing enhancement
• Other

Other Information:

This also adds tests and removes the unused SQLQuoteIdentifier function.