Add an option to record progressive performance logs.  Progressive
logs contain complete information after each recorded sample, by
writing partial address maps at each sample, containing all new
addresses introduced by the sample.  Furthermore, when recording a
progressive log, the output stream is flushed after each sample.

This allows recording complete logs even in cases where they can't
be properly terminated, such as when GIMP crashes or freezes in the
middle of the log.

Progressive logs are disabled by default, since they potentially
increase the sampling cost.  They can be enabled through a toggle
in the log file-dialog, or through the
GIMP_PERFORMANCE_LOG_PROGRESSIVE environment varaible.