start adding tracing spans to internals (#478)

We've adopted `tracing` for diagnostics, but currently, it is just being
used as a drop-in replacement for the `log` crate. Ideally, we would
want to start emitting more structured diagnostics, using `tracing`'s
`Span`s and structured key-value fields.

A lot of the logging in `h2` is already written in a style that imitates
the formatting of structured key-value logs, but as textual log
messages. Migrating the logs to structured `tracing` events therefore is
pretty easy to do. I've also started adding spans, mostly in the read
path.

Finally, I've updated the tests to use `tracing` rather than
`env_logger`. The tracing setup happens in a macro, so that a span for
each test with the test's name can be generated and entered. This will
make the test output easier to read if multiple tests are run
concurrently with `--nocapture`.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

tests/h2-support/src/prelude.rs

@@ -28,7 +28,7 @@ pub use super::assert::assert_frame_eq;
 
 // Re-export useful crates
 pub use tokio_test::io as mock_io;
-pub use {bytes, env_logger, futures, http, tokio::io as tokio_io};
+pub use {bytes, futures, http, tokio::io as tokio_io, tracing, tracing_subscriber};
 
 // Re-export primary future types
 pub use futures::{Future, Sink, Stream};