net: switch from log to tracing (#1455)

* net: switch from `log` to `tracing`.

Motivation:

The `tracing` crate implements scoped, structured, context-aware
diagnostics, which can add significant debugging value over unstructured
log messages. `tracing` is part of the Tokio project. As part of the
`tokio` 0.2 changes, I thought it would be good to move over from `log`
to `tracing` in the tokio runtime.

Solution:

This branch replaces the use of `log` in `tokio-net` with
`tracing`. I've tried to leave all the instrumentation points more or
less the same, but modified to use structured fields instead of string
interpolation.

Notes:

I removed the timing in `Reactor::poll` in favor of simply adding a
`#[tracing::instrument]` attribute. Since the generated `tracing` span
will have enter and exit events, a `tracing::Subscriber`
implemementation can use those to record timestamps, and process that
timing data in a much more sophisticated manner than including it in a
log line.

We can add the timestamps back if they're desired.

Signed-off-by: Eliza Weisman <[email protected]>

tokio-net/tests/process_stdio.rs

@@ -2,7 +2,7 @@
 #![warn(rust_2018_idioms)]
 
 #[macro_use]
-extern crate log;
+extern crate tracing;
 
 use std::env;
 use std::io;