On each shard, we register a listener for the new direct failure detector service.
The listener maintains a set of live addresses; on mark_alive it adds a
server to the set and on mark_dead it removes it. This set is then used
to implement the `raft::failure_detector` interface, consisting of
`is_alive()` function, which simply checks set membership.
There is some complexity in between, because we need to translate
direct_failure_detector endpoint_ids to inet_addresses and raft::server_ids
to inet_addreses, but all building blocks are already there.
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