PubSub channel subscriber accounting was incorrect; the counter
was not decremented when unsubscribing.
The handler count, reflecting the current number of subscriptions,
was maintained in the AbstractSubscribableChannel. The count really
belongs in the dispatcher and calls should be delegated to it, particularly
for a custom ASC, which might have a custom AbstractDispatcher.
However, it is possible (although perhaps rare) that someone
could implement a custom channel with a custom dispatcher that
is *not* a subclass of AbstractDispatcher.
Therefore, we now delegate to the dispatcher if we can, and
revert to a counter in the channel if we can't.
Also, the asymmetric accounting in this case is resolved; the
counter is decremented when a subscriber unsubscribes.
Polishing - factor out common code.
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