Replace Journal public data members with member function accessors
in order to make Journal lighter weight. The change makes a
Journal cheaper to pass by value.
Also add missing stream checks (e.g., calls to JLOG) to avoid
text processing that ultimately will not be stored in the log.
Class io_list manages children that perform asynchronous
I/O operations. The treatment of close and destruction is
refactored to fix race conditions during exit.
