DNSE-744 — Incremental zone transfer response does not end with proper SOA
Incremental zone transfer responses must contain the exact number of current zone SOA records. It may be
one SOA (as a single RR in response) in case of "no change" at server
two SOA RRs in case of full-zone (AXFR-style) reply
three SOA RRs in case of IXFR response with exactly one difference block for current serial.
The proxy received a response that does not fulfill this criteria.
Whole answer is ignored and next server is tried.