Fix for longstanding issue of chunks which didn't exist in a region file still being unnecessarily re-wiped by the Trim process, which also added to the reported number of individual trimmed chunks. Now chunks which were already nonexistent aren't "trimmed" for no reason and the reported number of individual trimmed chunks is exactly correct.
Added check to Trim process to make sure it doesn't drag on for too long per tick; shouldn't normally be possible on Trim process as opposed to Fill process, but I've had a couple of reports indicating it might rarely be an issue.
