Add index to database session handler for session expiry

[Deltik]

Fixes: #4574

Motivation and Context

Adding an index on the session expiry field in the session table greatly improves garbage collection performance

Description

Add an index for e107_session.session_expiry

How Has This Been Tested?

Manual acceptance test: Observed that site loads faster after adding the new index on a site with a million rows in the e107_session table

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (a change to man pages or other documentation)

Checklist

• My code adheres to the e107 code style standard.
• I have read the contributing document.
• I have added tests to cover my changes.
• All new and existing tests pass.

[codeclimate]

Code Climate has analyzed commit 54ed823 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 0.0% (80% is the threshold).

This pull request will bring the total coverage in the repository to 34.4% (0.1% change).

View more on Code Climate.

[CaMer0n]

Thank you! 👍