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The `view_update_write_response_handler` class, which is a subclass of `abstract_write_response_handler`, was created for a single purpose: to make it possible to cancel a handler for a view update write, which means we stop waiting for a response to the write, timing out the handler immediately. This was done to solve issue with node shutdown hanging because it was waiting for a view update to finish; view updates were configured with 5 minute timeout. See #3966, #4028. Now we're having a similar problem with hint updates causing shutdown to hang in tests (#8079). `view_update_write_response_handler` implements cancelling by adding itself to an intrusive list which we then iterate over to timeout each handler when we shutdown or when gossiper notifies `storage_proxy` that a node is down. To make it possible to reuse this algorithm for other handlers, move the functionality into `abstract_write_response_handler`. We inherit from `bi::list_base_hook` so it introduces small memory overhead to each write handler (2 pointers) which was only present for view update handlers before. But those handlers are already quite large, the overhead is small compared to their size. Not all handlers are added to the cancelling list, this is controlled by the `cancellable` parameter passed to the constructor. For now we're only cancelling view handlers as before. In following commits we'll also cancel hint handlers.