Restrict the number of scheduled jobs log entries shown

Right now, the View Log button for scheduled jobs wlll show the entire log table OR the entire log table for a specific job. For most sites this log table is quite huge and will pretty much cause. civicrm.org had more than 260K entries.

We should limit the number of entries shown to prevent an out of memory error.

In future releases would be good to have:

a. A pager to page through the rest of the entries
b. A scheduled job to clean up this table on a periodic basis. For most orgs the data in this table is not very useful after a few days