mod diagnostics improvements

[tgeoghegan]

With #3129, we now have the capability of dumping descriptions of events to files. This could be useful for debugging a variety of things where logs aren't suitable so I think we will want to hang on to this feature indefinitely. The module I cobbled together could use some improvements:

• fix the todo!()s in the deserialize() implementations
• see if we can unify the nearly identical serialize impls for AggregationJobId and TaskId (we might consider adding a serde feature to janus_messages that derives Serialize/Deserialize right there
• make writing of events even more async. Right now, whatever path in a request handler triggered the writing of an event will block until the whole thing is written to a file. It'd be nice if we could ship that off to some async context and let the request handler return whatever error the client is going to get.