Add self cleanup for too many logs for any one operation.

Problem/Motivation

When a BatchOperation runs on cron and has a cron timing that makes it run often, it is easy to have the number of BatchOpLogs growing rapidly.

We may want to have a setting to define how many BatchOpLogs are retained for a given BatchOperation maybe defaulting to 100 but allowing setting whatever value desired.

Proposed resolution

1. Add a setting field for the number of logs to retain (default to 100), if 0 it will retain ALL of them
2. add section to hook_cron that every week checks the number of logs for each Batch operation and deletes any that that are above the limit.

Remaining tasks

User interface changes

API changes

Data model changes