Hibernate Search doesn't properly execute the backend's preStop() operation on shutdown

org.hibernate.search.engine.backend.spi.BackendImplementor#preStop is never called from anywhere.

Fortunately it does not change much, since the current preStop() implementations mostly change some internal state to prevent users from submitting more work, then return an already-completed future. So generally this won't matter.

Still, we might as well implement this correctly.