Remove metrics records older than 2 days

[rnagle]

This PR adds a cleanup task to the analytic bridge cron which, after the plugin has pulled in the most recent metrics records from Google, runs a query to delete any records that we no longer need.

This is in an effort to keep the overall size of the analytic bridge tables to a minimum.

[rnagle]

Before deploying to any sites using the current version of the plugin, it may be wise to run an initial delete of old records by running the following SQL against the database directly:

delete wp_analyticbridge_metrics from wp_analyticbridge_metrics where startdate < (curdate()  - interval 2 day);