Delete old click/visit entity records on cron

Problem/Motivation

When a user visits the site using an affiliate url, an affiliate_click entity is created for each visit.
This can cause the entity table to grow quite large on busy sites.

Proposed resolution

provide a setting to delete click entities older than a configurable timeframe.

Also it may be useful to make the saving of clicks/visits optional. They are nice to have for reporting, but not required to create conversions.