MemLog: make [log_rep_empty] have a synced LogDescriptor again and reprove [flush_unsync_ok]
MemLog: make [log_rep_empty] have a synced LogDescriptor again and reprove [flush_unsync_ok]

mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl