add optional support for tokio-console

This turned out to be quite hairy, mostly because we need to apply the
config's log level filter to the actual logs (stdout and, optionally
sentry), but do not want to filter out the tokio tracing events needed by
the console_subscriber. I hit several edge cases in tracing getting
this to work, and we now depend on a git version of tracing with a
backported patch :(

docs/development.md

@@ -2,3 +2,21 @@
 
 Information about developing the project. If you are only interested in using
 it, you can safely ignore this section.
+
+## Debugging with `tokio-console`
+
+[`tokio-console`][1] can be a useful tool for debugging and profiling. To make
+a `tokio-console`-enabled build of Conduwuit, enable the `tokio_console` feature,
+disable the default `release_max_log_level` feature, and set the
+`--cfg tokio_unstable` flag to enable experimental tokio APIs. A build might
+look like this:
+
+```bash
+RUSTFLAGS="--cfg tokio_unstable" cargo build \
+    --release \
+    --no-default-features \
+    --features
+    backend_rocksdb,systemd,element_hacks,sentry_telemetry,gzip_compression,brotli_compression,zstd_compression,tokio_console
+```
+
+[1]: https://docs.rs/tokio-console/latest/tokio_console/