Switch zebrad to a non-blocking tracing logger

[teor2345]

Motivation

Some of our tests might be deadlocking when the log buffers fill up.
This could happen to users as well.

This is a high priority, because it is causing test failures.

API Reference

https://docs.rs/tracing-appender/0.2.2/tracing_appender/#non-blocking-writer

Designs

• Use tracing-appender to make stdout writes non-blocking
• Use a lossy logger, so we drop any output we can't write, rather than blocking