Handle client close/cancel on grpc mutiny streaming service

[pcasaes]

When using the Mutiny GRPC interface for streaming as described here, we eventually see the following error whenever a client closes a connection before the service tries to push down new messages:

2022-04-21 13:17:10,859 ERROR [io.qua.mut.run.MutinyInfrastructure] (vert.x-eventloop-thread-0) Mutiny had to drop the following exception: io.grpc.StatusRuntimeException: CANCELLED: call already cancelled. Use ServerCallStreamObserver.setOnCancelHandler() to disable this exception
        at io.grpc.Status.asRuntimeException(Status.java:526)
        at io.grpc.stub.ServerCalls$ServerCallStreamObserverImpl.onNext(ServerCalls.java:366)
        ...

What is happening is that the Multi subscription isn't being canceled whenever a client disconnects. Besides the log pollution this behavior can cause some user pain, particulary if the user adds a termination handler to the multi subscription.

.onTermination().invoke(handleTermination)

If the service pushes messages downstream infrequently we might also see a situation where long disconnected client's subscriptions are still being maintained, possibly causing observability and memory pressure concerns.

The solution here is to simply add a cancel and close handler as the error message suggests. On either on cancel or on close we'll cancel the multi subscription.

[cescoffier]

LGTM, @michalszynkiewicz wdyt?