[RND-643] Serializable Snapshot Isolation (SSI) in PostgreSQL

[jleiva-gap]

• Add documentation about PostgreSQL Serializable Snapshot Isolation (SSI) performance testing
• Add a note to locking\delete.test.ts

Description

Types of changes

• Bug fix (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 change)

Checklist

• I have signed all of the commits on this PR.
• I have agreed to the Ed-Fi Individual Contributors' License
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.