renaming GNUNET_TIME_relative_get_forever and GNUNET_TIME_absolute_get_forever method...

renaming GNUNET_TIME_relative_get_forever and GNUNET_TIME_absolute_get_forever methods, adding underscore, to make it clear that the respective #defines should be used instead; replacing use of direct function calls with respective macros where applicable; adding additional GNUNET_TIME_relative_get_xxx-functions to avoid calls to GNUNET_TIME_relative_multiply, which turn out to have gotten performance-relevant