Add index to civicrm_cache for improved cleanup times

CRM-10264 calls cache cleanup more frequently, and I believe the performance of this query can be improved with some additional indexing.

On a quick test, I was able to reduce the query time from 1s to .22s with this index for a simulated cache cleanup.

Proposed fix -

ALTER TABLE civicrm_cache ADD INDEX civicrm_cache_idx (group_name, path, created_date)

Ugly bash script to reproduce unscientific testing is attached