Add a structured logger

[karlseguin]

In debug mode, it has a more user-friendly output:

level | the log messge | ms since last message | key=value key=value

In release mode, it logs using logfmt, which is supported by most log ingestion frameworks.

Not being used anywhere right now, keeping this PR small with no impact on existing code.

[krichprollsch]

@karlseguin LGTM. Can you please rebase the branch? :pray: