"OPTIMIZE TABLE" run on schedule can cause too much load on DB server

[justingit]

This is mostly because it's run every "x"minutes, rather than every week or few weeks.

It would be better to only optimize the table when there has been a very large amount of table rows that have been deleted since the last optimization. (we optimize mostly to save disk space)

We could keep track of rows deleted in the settings db under a new setting.