base: Reimplement the DPRINTF mechanism in a Logger class

base: Reimplement the DPRINTF mechanism in a Logger class

This patch adds a Logger class encapsulating dprintf.  This allows
variants of DPRINTF logging to be constructed and substituted in
place of the default behaviour.

The Logger provides a logMessage(when, name, format, ...) member
function like Trace::dprintf and a getOstream member function to
use a raw ostream for logging.

A class OstreamLogger is provided which generates the customary
debugging output with Trace::OstreamLogger::logMessage being the
old Trace::dprintf.