The `tracing` crate seems to be the go-to
logging/profiling/instrumentation solution nowadays. Perhaps in the
future we could use `tracing` for profiling instead of (or in addition
to) the `perf`-based `cargo flamegraph` command. This would sidestep the
issue of `rayon` polluting the output. I conducted an initial experiment
by adding some more spans but wasn't very happy with the result.
Log messages have also been improved. There is some additional context
and events are raised when clients are added/removed from the server.
Adds the performance_tests/ directory.
In the future we could use our own fake client software instead of
rust-mc-bot. This would make it easier to run the tests.
