When inline signing is enabled for a zone without creating signing keys
for it, changes subsequently applied to the raw zone will not be
reflected in the secure zone due to the dns_update_signaturesinc() call
inside receive_secure_serial() failing. Given that an inline zone will
be served (without any signatures) even with no associated signing keys
being present, keep applying raw zone deltas to the secure zone until
keys become available in an attempt to follow the principle of least
astonishment.
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