switch from log to tracing

I think this is a big improvement in readability.

I removed the `lnav` config, which is a little sad, but I don't think it
supports this structured logging format well. Still seems worthwhile on
balance.

server/db/upgrade/v6_to_v7.rs

@@ -5,9 +5,9 @@
 /// Upgrades a version 6 schema to a version 7 schema.
 use failure::{format_err, Error, ResultExt};
 use fnv::FnvHashMap;
-use log::debug;
 use rusqlite::{named_params, params};
 use std::{convert::TryFrom, path::PathBuf};
+use tracing::debug;
 use url::Url;
 use uuid::Uuid;