[meta] IXFR processing is slow

Umbrella ticket for all the IXFR tickets:

• #1447 (closed) - "BIND unresponsive during large IXFR and RPZ transfers"
• #1516 - "Option on slave servers to apply inbound IXFR to a temporary version of the zone being updated instead of incrementally to the live zone"
• #1518 (closed) - "When handling large inbound IXFRs, make appropriate use of an iterator to avoid client query processing starvation"
• #1526 (closed) - "Add a new option for maximum number of changes permitted when generating a .jnl file from ixfr-from-differences"
• #1550 - "rndc stop issued after server (or single zone) rndc reload and during ixfr-from-differences processing leaves the .jnl file corrupted"
• #1834 (closed) - "IXFR of a new NSEC3 chain causes temporary performance drop"

Just to recap:

1. There has been several reports that IXFR processing of NSEC3 related changes (like when operator of TLD changes salt), the query rate drops significantly.
2. There has been also a report that IXFR processing of large RPZ updates makes the query processing very slow or even stuck.