Support for outgoing incremental zone transfers (IXFR)

[bal-e]

Cascade does not currently support IXFR queries in its DNS servers (it does support consuming IXFRs from upstream servers). Outgoing IXFR support would reduce network bandwidth and improve efficiency -- it may even be useful for reviewing zones. This could be implemented in two ways:

• Maintain the existing infrastructure within Cascade and internally diff consecutive published zones to generate IXFRs. This would improve network bandwidth but slow down zone publishing.

• Wait for Cascade to implement incremental signing (Incremental signing #299), which would improve the internal infrastructure and make fully incremental operation (including efficiently computing IXFR output) possible.

This also relates to #295 -- zones with multiple versions could be persisted using IXFR outputs, which would save disk space and improve efficiency.

[ximon18]

Maintain the existing infrastructure within Cascade and internally diff consecutive published zones to generate IXFRs. This would improve network bandwidth but slow down zone publishing.

Related: #417 which introduces automatic diff generation.

Additional work still needed to implement IXFR out after that PR is merged includes:

• Making generated diffs available to XfrMiddlewareSvc used by src/units/zone_server.rs.
  • Making the RPR Overhaul how zone data is stored #417 generated diffs compatible with XfrMiddlewareSvc which requires diffs to implement the domain crate ZoneDiff trait, which is not the case in PR Overhaul how zone data is stored #417. Alternatively don't use XfrMiddlewareSvc to do XFR out and instead write PR Overhaul how zone data is stored #417 compatible code to do so.
• Persist generated diffs to disk and reload them on startup, IIRC PR Overhaul how zone data is stored #417 doesn't do this.