[fix][broker] Avoid blocking the dispatcher close path on delayed-delivery tracker close

[merlimat]

Motivation

BucketDelayedDeliveryTracker.close() blocks up to AsyncOperationTimeoutSeconds (60s) on FutureUtil.waitForAll(snapshotCreateFutures).get(...), hidden behind the void close() of DelayedDeliveryTracker. It is invoked by PersistentDispatcherMultipleConsumers.close() (and the Classic variant) via delayedDeliveryTracker.ifPresent(DelayedDeliveryTracker::close), which runs inside the topic/subscription close chain on a metadata / CompletableFuture-callback thread (e.g. the ExtensibleLoadManager getAssignedBrokerLookupData(...).thenAccept(...) continuation during unload). A pending snapshot generation could therefore stall that callback thread for up to 60s.

Modifications

• Add CompletableFuture<Void> closeAsync() to the DelayedDeliveryTracker interface as a default that delegates to close(). Because the interface extends AutoCloseable, close() must stay void; the default keeps InMemoryDelayedDeliveryTracker and the DISABLE no-op working unchanged.
• BucketDelayedDeliveryTracker overrides closeAsync() to perform the cleanup (super.close() + the bucket queue closes) under its monitor and then return waitForAll(snapshotCreateFutures) (with .exceptionally(...) logging, as before) instead of blocking on get(60s). Its AutoCloseable close() now delegates to closeAsync().join() for synchronous callers (try-with-resources, tests).
• PersistentDispatcherMultipleConsumers and its Classic variant chain closeAsync() into the returned close future, so the tracker still finishes closing before consumers are disconnected — but without blocking the callback thread.

Verifying this change

This change is covered by existing tests, which pass: BucketDelayedDeliveryTrackerTest, BucketDelayedDeliveryTrackerThreadSafetyTest, InMemoryDeliveryTrackerTest, and DelayedDeliveryTrackerFactoryTest.

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