Bug: Cluster synchronisation of already-deleted session files

Cluster synchronisation could loop with a failed copy transaction when, while processing a backlog of synchronisation transactions, a copy transaction for a session ({\tt .hds}), active session ({\tt .hda}), or remember me ({\tt .hdr}) file was executed after the file in question had been deleted at the close of the session. I added code, similar to the September 5 fix for deletion transactions, which considers copy transactions which fail due to nonexistence of the source file as having completed normally.