log the reason for falling back to AXFR from IXFR at level info

Evan Hunt requested to merge 2683-ixfr-logging into main

messages indicating the reason for a fallback to AXFR (i.e, because the requested serial number is not present in the journal, or because the size of the IXFR response would exceeed "max-ixfr-ratio") are now logged at level info instead of debug(4).

Closes #2683 (closed)

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