This add the infrastructure to let other middleware report their health
status back to the health middleware. A health.Healther interface is
introduced and a middleware needs to implement that. A middleware
that supports healthchecks is statically configured.
Every second each supported middleware is queried and the global health
state is updated.
Actual tests have been disabled as no other middleware implements this
at the moment.
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.
