Optimize startup time: Make Actuator features lazy

[dsyer]

If the Actuator context and all its features were only available after the app had finished starting up and was ready to serve traffic for business requests, it might not matter a lot of the time, especially in dev mode. Benchmarks show that it might be worth it (you save 700ms on a fast machine, out of a Petclinic startup time of 3-4s).