We want to replace the current
`storage_service::register_client_shutdown_hook()` machinery with
something more powerful. We want to register all running client protocol
servers with the storage service, allowing enumerating these at runtime,
checking whether they are running or not and starting/stopping them.
As the first step towards this, we introduce an abstract interface that
we are going to implement at the controllers of the various protocol
servers we have. Then we will switch storage service to collect pointers
to this interface instead of simple stop functors.
