Clean up config settings and dead code.
This is mostly about cleaning up the config format, to bring it into line with our conventions. In particular:
* There should be a blank line after `## Section ##' headings
* There should be a blank line between each config setting
* There should be a `#`-only line between a comment and the setting it describes
* We don't really do the `# #` style commenting-out of whole sections if we can help it
* rename `tracer_enabled` to `enabled`
While we're here, do more config parsing upfront, which makes it easier to use
later on.
Also removes redundant code from LogContextScopeManager.
Also changes the changelog fragment to a `feature` - it's exciting!
