Add cache expiration for tags

[markdboyd]

After #6, we are caching the tags for each log group instead of looking them up when processing every log to get a speed boost.

While this works well, we need to add some expiration of the tags so that if the tags change, those changes will get picked up by the ingestion.

Acceptance Criteria

• Add tag cache expiration logic

[markdboyd]

This is deployed to prod and verified there