Increase the logging level of some IXFR messages

(Originally reported as #2671 (comment 211777).)

I'd like to suggest that we increase the logging level of these messages:

                        xfrout_log1(client, question_name, question_class,
                                    ISC_LOG_DEBUG(4),
                                    "IXFR version not in journal, "
                                    "falling back to AXFR");
[...]
                        xfrout_log1(client, question_name,
                                    question_class, ISC_LOG_DEBUG(4),
                                    "IXFR delta size (%zu bytes) "
                                    "exceeds the maximum ratio to "
                                    "database size "
                                    "(%" PRIu64 " bytes), "
                                    "falling back to AXFR",
                                    jsize, dbsize);

...perhaps all the way to ISC_LOG_INFO. It's not helpful for this to be a mysterious black box. Spurious AXFR fallbacks could be caused by a still-undetected corruption in the journal file, or a computational error counting the size of the database or the journal transactions, and without debug logging, or a full before-and-after copy of the database and journal, it's difficult to sort out which it is.