Add a new option for maximum number of changes permitted when generating a .jnl file from ixfr-from-differences

Inspired by support ticket #15864, #15857 and others.

It can be challenging to manage zone content in production where you want to ensure that if a zone has been significantly modified, that the resulting propagation takes place using AXFR instead of IXFR.

One point of pain is on the provisioning master, where there is currently no automation that supports this - the only way to force AXFR instead of IXFR is to remove the .jnl file so that IXFR requests can only be satisfied by an AXFR (ref #1515 (closed)).

In quite a few instances, provisioning masters have their zone content updated by external tools that update the zone on disk, followed by a reload and the application of 'ixfr-from-differences yes;' to generate the .jnl from the diff between the before and after versions of the zone.

This is a request to add an option to 'ixfr-from-differences' that limits the size of the journal file being created - if the diff between the two versions of the zone exceeds the configured threshold, then don't create the .jnl file, so that onward zone propagation is forced to AXFR instead of IXFR.