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>

Cargo.toml

@@ -48,6 +48,7 @@ tokio = { version = "0.2", features = ["io-util"] }
 bytes = "0.5.2"
 http = "0.2"
 tracing = { version = "0.1.13", default-features = false, features = ["std", "log"] }
+tracing-futures = { version = "0.2", default-features = false, features = ["std-future"]}
 fnv = "1.0.5"
 slab = "0.4.0"
 indexmap = "1.0"