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.
The Journal API is affected. There are two uses for the
Journal::Severity enum:
o It is used to declare a threshold which log messages must meet
in order to be logged.
o It declares the current logging level which will be compared
to the threshold.
Those uses that affect the threshold are now named threshold()
rather than severity() to make the uses easier to distinguish.
Additionally, Journal no longer carries a Severity variable.
All handling of the threshold() is now delegated to the
Journal::Sink.
Sinks are no longer constructed with a default threshold of
kWarning; their threshold must be passed in on construction.
