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For counter updates, use a counter ID that is constructed from the node's rack instead of the node's host ID. A rack can have at most two active tablet replicas at a time: a single normal tablet replica, and during tablet migration there are two active replicas, the normal and pending replica. Therefore we can have two unique counter IDs per rack that are reused by all replicas in the rack. We construct the counter ID from the rack UUID, which is constructed from the name "dc:rack". The pending replica uses a deterministic variation of the rack's counter ID by negating it. This improves the performance and size of counter cells by having less unique counter IDs and less counter shards in a counter cell. Previously the number of counter shards was the number of different host_id's that updated the counter, which can be typically the number of nodes in the cluster and continue growing indefinitely when nodes are replaced. with the rack-based counter id the number of counter shards will be at most twice the number of different racks (including removed racks, which should not be significant). Fixes SCYLLADB-356