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Draining hints may occur in one of the two scenarios: * a node leaves the cluster and the local node drains all of the hints saved for that node, * the local node is being decommissioned. Draining may take some time and the hint manager won't stop until it finishes. It's not a problem when decommissioning a node, especially because we want the cluster to retain the data stored in the hints. However, it may become a problem when the local node started draining hints saved for another node and now it's being shut down. There are two reasons for that: * Generally, in situations like that, we'd like to be able to shut down nodes as fast as possible. The data stored in the hints won't disappear from the cluster yet since we can restart the local node. * Draining hints may introduce flakiness in tests. Replaying hints doesn't have the highest priority and it's reflected in the scheduling groups we use as well as the explicitly enforced throughput. If there are a large number of hints to be replayed, it might affect our tests. It's already happened, see: scylladb/scylladb#21949. To solve those problems, we change the semantics of draining. It will behave as before when the local node is being decommissioned. However, when the local node is only being stopped, we will immediately cancel all ongoing draining processes and stop the hint manager. To amend for that, when we start a node and it initializes a hint endpoint manager corresponding to a node that's already left the cluster, we will begin the draining process of that endpoint manager right away. That should ensure all data is retained, while possibly speeding up the shutdown process. There's a small trade-off to it, though. If we stop a node, we can then remove it. It won't have a chance to replay hints it might've before these changes, but that's an edge case. We expect this commit to bring more benefit than harm. We also provide tests verifying that the implementation works as intended. Fixes scylladb/scylladb#21949 Closes scylladb/scylladb#22811