Panic on receipt of NOTIFY in IXFR-in handler.

[ximon18]

Given an NSD primary, with Cascade HEAD of main as secondary having a zone whose source is the primary, when nsd-control notify <zonename> is issued Cascade panics with:

thread 'cascade-worker' (3235710) panicked at src/loader/server.rs:231:14:
internal error: entered unreachable code
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

The code referred to at server.rs:231 says:

    => unreachable!(),