It was passed to `raft_group_registry::direct_fd_proxy` by value. That
is a bug, we want to pass a reference to the instance that is living
inside `gossiper`.
Fortunately this bug didn't cause problems, because the pinger is only
used for one function, `get_address`, which looks up an address in a map
and if it doesn't find it, accesses the map that lives inside
`gossiper` on shard 0 (and then caches it in the local copy).
Explicitly delete the copy constructor of `direct_fd_pinger` so this
doesn't happen again.
Closes#11661
67ee6500e3