Closes #14

Changes

• General cleanup (phpdoc, code style)
• Made scheduled task spawn adhoc task to do purge, so it is more robust and doesn't block other scheduled tasks if it fails.
• Improve query:
  • Loop and query only 1k records per loop.
  • Only query the id since that's all it uses
  • Pass through 1k records to delete() instead of 1 at a time. This massively improves the speed at which it runs (since it can delete records in chunks of 1k instead of just 1 at a time)

Comparison
Scenario: Testing with ~450k rows in mdl_local_mail_log each with an attachment set.

Before:

• Started running, but eventually died:

Deleting log entries older than 7 days.
Fatal error: Allowed memory size of 536870912 bytes exhausted (tried to allocate 20480 bytes) in /var/www/site/lib/dml/pgsql_native_moodle_database.php on line 1040

Fatal error: Allowed memory size of 536870912 bytes exhausted (tried to allocate 20480 bytes) in /var/www/site/admin/tool/excimer/classes/sample_set.php on line 84

After:

• Completes successfully. Takes ~2-3 minutes to process the entire 450k records.
• Uses ~10mb of memory (according to excimer)