This still needs cleanup, but this is a first pass the cleans some
cruft and documents our style (in middleware.md) and makes all the
docs match that style.
Make middleware that sets up a (http) handler survive a graceful
restart. We calls the middleware's Shutdown function(s). If restart
fails the Start function is called again.
* middleware/health: OK
* middleware/pprof: OK
* middleware/metrics: OK
All restart OK.
