Add a data retention cron

[andrew-kb]

This provides a simple, data driven interface to purging old data from tables.

I've moved exception log and worker job clean-up into retention jobs, which in the former case avoids a potentially lengthy operation (delete on MyIASM tables grows increasingly slow the more operations performed sans OPTIMIZE TABLE) occurring during a request.

Added new jobs for rate_limit_hits and login_attempts because we weren't cleaning these up.