This splits diag.rs, which was growing way too big for my taste, into a
number of submodules. This should help us compartmentalize tests better,
as well as use mod namespaces to shorten our struct/enum names.
A few minor refactors, and a more major one that renames
RecordingStore's update_entry_qmdl_size to
update_current_entry_qmdl_size, since the only time we're ever updating
an entry's QMDL size is when it's the current one.
Major changes:
* QmdlWriter now outputs gzipped QMDL files by default
* QmdlReader renamed to QmdlMessageReader, and reads both compressed and
uncompressed QMDL. It no longer requires bounding to avoid reading
partially written files.
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.
Will Greenberg