REP-712 Improve HealthCheckService API

These changes are branched off of the spring-refactor branch since so many of the same things are touched. If desired, I'd be okay with rebasing and pulling this stuff in after the spring stuff is taken care of.
Since the implementation of the proxy service is an inner class, the methods on the HealthCheckService should be private. (or if that won't work, protected, but I think private will work.)

We don't want people touching the health check service directly, but operating through the proxy object they receive.