Can be used to show statistics about listeners to users and other
debug information.

* Add a gzip encoding handler
* Add cache-friendly headers to static content
* Refactor the status page handler to look like a http.Handler
* Update jquery to current version

Remove a lot of boilerplate by using the errgroup package instead of
our own custom cumbersome solutions. Control server lifetime with an
outer Context.

Causes a slight reduction in boilerplate, and gets rid of a bunch of
custom logic for command-line parsing that is no longer needed.

Fix also some errors in the status template, and update to Bootstrap
4.3.1 and JQuery 3.4.0.

Also add instrumentation for the load balancer module.