Log cleaner must handle transactions spanning multiple segments

When cleaning transactions, we have to do some bookkeeping to keep track of which transactions still have data left around. As long as there is still data, we cannot remove the transaction marker. The problem is that we do this tracking at the segment level. We do not carry over the ongoing transaction state between segments. So if the first entry in a segment is a marker, we incorrectly clean it. In the worst case, data from a committed transaction could become aborted.