Delete handling in prune changes job assumes lower ID values are further back in time

[JamesDPC]

While this is probably OK to assume for nearly all cases, there could exist a situation where an ID of 500 is more recent than an ID of 1000.

The query should instead just delete records prior to the pruneBefore value. This would require the Created column in the DataChangeRecord table have an index applied to it.

silverstripe-datachange-tracker/src/Job/PruneChangesBeforeJob.php

Line 59 in 66489cb

$query = new SQLDelete('DataChangeRecord', '"ID" < \'' . $max . '\'');

I'll knock up a PR for review, at some point.