There is a data race between the statistics channel, which uses
`dns_zone_getxfr()` to get a reference to `zone->xfr`, and the creation
of `zone->xfr`, because the latter happens outside of a zone lock.
Split the `dns_xfrin_create()` function into two parts to separate the
zone tranfer startring part from the zone transfer object creation part.
This allows us to attach the new object to a local variable first, then
attach it to `zone->xfr` under a lock, and only then start the transfer.
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