LDEV-5083 Prevent WF from freezing on shutdown because of unclosed Flux
Open Fluxes prevent WF from shutting down as they are threads not
managed by the server lifecycle.
Now we force complete Sinks on Spring context destroy. It closes
underlying Flux threads.
The problem is that it occurs when most Spring beans are already
destroyed. Fluxes on close want to emit a signal to front end
subscribers. Spring tries to reinitialise controllers which should send
those signals. It fails as context basically does not exist anymore.
All in all we get some harmless errors on server shutdown.
The way to avoid would be to have more control over Spring bean destroy
order which does not seem easily achievable.
