Replace borg connection manager with thread safe db pool

[daminichopra]

Proposed changes

Does changes as per references Fix for Issue

Types of changes

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

Checklist

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• [ x ] I have signed the CLA
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added necessary documentation (if appropriate)
• [ ] Any dependent changes have been merged and published in downstream modules

Further comments

• Replaced "borg" connection used in file connection_manager.py with CrateDB's SQLAlchemy dialect

• Also, took references from discussion

• used create_engine connection.

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