Previously, the attempt was made to enable only those `tokio` features
that are actually used. However, due to default-features still being
enabled and `tauri` using the `full` feature, for most intents and purposes
that wasn't effective.
Now default features are disabled, and `full` isn't used anymore, forcing
all crates to mention the exact features they need.
Note that `tracing` is missing here, as it wasn't effective previously
without the `--cfg tokio_unstable` option also being specified as rustflag.
