ND-JSON (newline-delimited JSON) is just a file with a list of JSON
objects separated by newlines. This way, as the analyzer harness
processes new packets, it can simply append JSON-serialized results
to a file without parsing the entire thing first.
Also simplifies the analysis stuff to all operate in the diag thread.
Mixing async and sync I/O leads to a multitude of complications, and
generally speaking it's much more convenient to stick to one paradigm
or the other. Since axum (and many other HTTP servers) use async,
and since async is a convenient model for performing operations like
"handle an MPSC message or file read, whichever happens first", let's
commit to an async interface.
