Option on secondary servers to apply inbound IXFR to a temporary version of the zone being updated instead of incrementally to the live zone

This maybe needs to be an option that can be set globally, per view, by zone, and perhaps also per server.

Background: Support ticket #15857 and Support ticket #15075

Large inbound IXFRs can seriously degrade a BIND secondary nameserver's performance when serving clients (not necessarily from the same zones being transferred) at the same time. This is most likely because the updates are being applied directly to the active zone (unlike an inbound AXFR that creates a copy version of the zone and then swaps them when the inbound transfer is complete).

The overhead of maintaining state/status does not scale well with large incremental updates. IXFR was written many moons ago when memory was expensive and large incremental updates were uncommon. System memory is now easy to upgrade and in the interests of performance, it would be helpful to be able to choose the mode of IXFR update.

Servers hosting large numbers of authoritative zones would benefit particularly from a feature like this.