Using only one instance of ScheduledExecutorService

[csviri]

Currently we are using ScheduledThreadPoolExecutor per controller. It would be better to control threading and related configurations if it would be shared between controllers. So there are no more such services created.

[csviri]

see also: #533

[lburgazzoli]

eventually the executor service can also been injected i.e. it can use the same thread pool as quarkus