Add index to log table

[jtpalmer]

Description

Adds a new index to themod_logger.log_table database table.

Motivation and Context

This index is intended to improve the performance of the queries used in the internal dashboard "Log Data" panels. They are of the form:

SELECT id, logtime, priority, message
FROM log_table
WHERE ident = 'xdmod-shredder'
AND priority IN ('0','1','2','3','4','5','6','7')
AND logtime >= '2020-05-29 00:00:00'
AND logtime <= '2020-06-04 23:59:59';

The existing indexes don't include the logtime column.

Tests performed

Tested manually. (See https://metrics-dev.ccr.buffalo.edu/internal_dashboard/)

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:

• My code follows the code style of this project as found in the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.