Forwarding CQL requests is not implemented yet, but we're already
prepared to return the target to forward to when trying to execute
strongly consistent requests. Currently, if we're not a replica
of the affected tablet, we redirect the request to the first replica
in the list.
This is not optimal, because this replica may be down or it may be
in another rack, making us perform cross-rack requests during forwarding.
Instead, we should forward the request to the replica from the same
rack and handle the case where the replica is down.
In this patch we change the replica selection for forwarding strongly
consistent requests, so that when the coordinator isn't a replica, it
redirects the request to the replica from the same rack.
If the replica from the same rack is down, or there is no replica in
our rack, we choose the next closest replica (preferring same-DC replicas
over other DCs). If no replica is alive, the query fails - the driver
should retry when some replica comes back up.
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