oplog: Give logged_object a virtual destructor
oplog: Give logged_object a virtual destructor

logged_object is an abstract base class, so it needs a virtual
destructor.  This may help with some memory leaks.

bowlofstew/sv6,bowlofstew/sv6,aclements/sv6,aclements/sv6,bowlofstew/sv6,bowlofstew/sv6,aclements/sv6,bowlofstew/sv6,aclements/sv6,aclements/sv6