The main executor for blocking tasks should participate in the graceful shutdown

[mkouba]

We need to register a specific ShutdownListener.

[mkouba]

It turns out there is a special shutdown hook for this executor with its own configuration: https://quarkus.io/guides/all-config#quarkus-core_quarkus.thread-pool.shutdown-timeout. Unfortunately, there was a bug and the shutdown hook did not work as expected.